diff --git a/README.md b/README.md
index 62ceea0c..6d112a44 100644
--- a/README.md
+++ b/README.md
@@ -321,7 +321,22 @@ For information on the transition triggers, refer to:
 * description of the `timeout`, `ttl timeout`, and `ttr timeout` options in
 the sections of the corresponding [queue types](#queue-types).
 
-![Task state diagram](./doc/images/statediagram.svg)
+```mermaid
+flowchart LR
+      INIT((" "))-->  |"put()"| READY
+      INIT((" "))--> |"put('my_task_data', {delay = delay})"| DELAYED
+      READY--> |"take()"| TAKEN
+      READY--> |"delete() / ttl timeout"| DONE
+      READY--> |"bury()"| BURIED
+      TAKEN--> |"release() / ttr timeout"| READY
+      TAKEN--> |"release\n(id, {delay = delay})"| DELAYED
+      TAKEN--> |"ack() / delete()"| DONE
+      TAKEN--> |"bury()"| BURIED
+      BURIED--> |"delete() /\nttl timeout"| DONE
+      BURIED--> |"kick()"| READY
+      DELAYED--> |"timeout"| READY
+      DELAYED--> |"delete()"| DONE
+```
 
 # Installing
 
diff --git a/doc/images/statediagram.svg b/doc/images/statediagram.svg
deleted file mode 100644
index cc5b5a4d..00000000
--- a/doc/images/statediagram.svg
+++ /dev/null
@@ -1,40 +0,0 @@
-<?xml version="1.0" encoding="UTF-8" standalone="no"?><svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" contentScriptType="application/ecmascript" contentStyleType="text/css" height="546px" preserveAspectRatio="none" style="width:552px;height:546px;" version="1.1" viewBox="0 0 552 546" width="552px" zoomAndPan="magnify"><defs><filter height="300%" id="filrphww2x0uf" width="300%" x="-1" y="-1"><feGaussianBlur result="blurOut" stdDeviation="2.0"/><feColorMatrix in="blurOut" result="blurOut2" type="matrix" values="0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 .4 0"/><feOffset dx="4.0" dy="4.0" in="blurOut2" result="blurOut3"/><feBlend in="SourceGraphic" in2="blurOut3" mode="normal"/></filter></defs><g><ellipse cx="74.5" cy="16" fill="#000000" filter="url(#filrphww2x0uf)" rx="10" ry="10" style="stroke:none;stroke-width:1.0;"/><g id="READY"><rect fill="#FEFECE" filter="url(#filrphww2x0uf)" height="50" rx="12.5" ry="12.5" style="stroke:#A80036;stroke-width:1.5;" width="61" x="263" y="103"/><line style="stroke:#A80036;stroke-width:1.5;" x1="263" x2="324" y1="129.2969" y2="129.2969"/><text fill="#000000" font-family="sans-serif" font-size="14" lengthAdjust="spacingAndGlyphs" textLength="41" x="273" y="120.9951">READY</text></g><g id="DELAYED"><rect fill="#FEFECE" filter="url(#filrphww2x0uf)" height="50" rx="12.5" ry="12.5" style="stroke:#A80036;stroke-width:1.5;" width="77" x="7" y="245"/><line style="stroke:#A80036;stroke-width:1.5;" x1="7" x2="84" y1="271.2969" y2="271.2969"/><text fill="#000000" font-family="sans-serif" font-size="14" lengthAdjust="spacingAndGlyphs" textLength="57" x="17" y="262.9951">DELAYED</text></g><g id="TAKEN"><rect fill="#FEFECE" filter="url(#filrphww2x0uf)" height="50" rx="12.5" ry="12.5" style="stroke:#A80036;stroke-width:1.5;" width="61" x="237" y="245"/><line style="stroke:#A80036;stroke-width:1.5;" x1="237" x2="298" y1="271.2969" y2="271.2969"/><text fill="#000000" font-family="sans-serif" font-size="14" lengthAdjust="spacingAndGlyphs" textLength="41" x="247" y="262.9951">TAKEN</text></g><g id="DONE"><rect fill="#FEFECE" filter="url(#filrphww2x0uf)" height="50" rx="12.5" ry="12.5" style="stroke:#A80036;stroke-width:1.5;" width="58" x="309.5" y="387"/><line style="stroke:#A80036;stroke-width:1.5;" x1="309.5" x2="367.5" y1="413.2969" y2="413.2969"/><text fill="#000000" font-family="sans-serif" font-size="14" lengthAdjust="spacingAndGlyphs" textLength="38" x="319.5" y="404.9951">DONE</text></g><g id="BURIED"><rect fill="#FEFECE" filter="url(#filrphww2x0uf)" height="50" rx="12.5" ry="12.5" style="stroke:#A80036;stroke-width:1.5;" width="69" x="370" y="245"/><line style="stroke:#A80036;stroke-width:1.5;" x1="370" x2="439" y1="271.2969" y2="271.2969"/><text fill="#000000" font-family="sans-serif" font-size="14" lengthAdjust="spacingAndGlyphs" textLength="49" x="380" y="262.9951">BURIED</text></g><ellipse cx="338.5" cy="524" fill="none" filter="url(#filrphww2x0uf)" rx="10" ry="10" style="stroke:#000000;stroke-width:1.0;"/><ellipse cx="339" cy="524.5" fill="#000000" rx="6" ry="6" style="stroke:none;stroke-width:1.0;"/><!--MD5=[5d051701840a39d81aec12b7f4902293]
-link *start to READY--><path d="M83.09,21.31 C111.87,35.77 206.08,83.09 258.14,109.24 " fill="none" id="*start-to-READY" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="262.74,111.55,256.4887,103.9394,258.2707,109.3082,252.9019,111.0902,262.74,111.55" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="28" x="184.5" y="69.0669">put()</text><!--MD5=[9253ea1a709093758374d071a38e58bb]
-link *start to DELAYED--><path d="M73.47,25.97 C69.55,60.03 55.48,182.31 48.91,239.33 " fill="none" id="*start-to-DELAYED" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="48.3,244.64,53.3284,236.1715,48.8867,239.6745,45.3837,235.2328,48.3,244.64" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="109" x="65.5" y="125.0669">put('my_task_data',</text><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="95" x="72.5" y="140.1997">{ delay = delay })</text><!--MD5=[06edf5781c49650175283c5a5b2f53cf]
-link READY to TAKEN--><path d="M289.03,153.07 C284.57,177.11 277.7,214.09 272.92,239.79 " fill="none" id="READY-to-TAKEN" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="272,244.78,277.5787,236.6635,272.9145,239.8643,269.7137,235.2002,272,244.78" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="32" x="283.5" y="203.5669">take()</text><!--MD5=[18941f88cdacfd8425ce9a95296032a4]
-reverse link READY to TAKEN--><path d="M257.95,141.24 C239.05,149.78 217.49,163.24 206.5,183 C199.59,195.43 200.66,202.03 206.5,215 C212.89,229.19 225.05,241.15 236.84,250.18 " fill="none" id="READY-backto-TAKEN" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="262.69,139.18,252.8416,139.0857,258.1017,141.1668,256.0205,146.4269,262.69,139.18" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="58" x="209" y="196.0669">release() /</text><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="61" x="207.5" y="211.1997">ttr timeout</text><!--MD5=[d763bd56abfa4080cd26135f3d743e9c]
-link READY to DONE--><path d="M324.02,129.47 C361.59,131.87 424.14,142.2 454.5,183 C500.73,245.12 491.71,294.87 445.5,357 C427.88,380.69 396.91,394.71 372.81,402.5 " fill="none" id="READY-to-DONE" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="367.78,404.07,377.5623,405.2135,372.5539,402.5835,375.1839,397.5752,367.78,404.07" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="52" x="489" y="267.0669">delete() /</text><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="59" x="485.5" y="282.1997">ttl timeout</text><!--MD5=[2c1b88c7df31c0379fb2cb86f0e265c0]
-link READY to BURIED--><path d="M324.25,135.77 C349.07,142.98 382.55,157.07 399.5,183 C410.25,199.45 411.28,221.67 409.74,239.41 " fill="none" id="READY-to-BURIED" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="409.19,244.75,414.0949,236.2094,409.7045,239.7765,406.1373,235.3862,409.19,244.75" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="35" x="410.5" y="203.5669">bury()</text><!--MD5=[5cee283b5512138bf3513e6906d8d0cb]
-reverse link READY to BURIED--><path d="M315.35,157.37 C328.37,174.2 345.28,195.9 360.5,215 C368.34,224.84 377.07,235.59 384.67,244.9 " fill="none" id="READY-backto-BURIED" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="312.13,153.21,314.4669,162.7776,315.1868,157.1667,320.7977,157.8867,312.13,153.21" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="29" x="361.5" y="203.5669">kick()</text><!--MD5=[eb9eca0627440ee2af82c7be1b0a8840]
-reverse link DELAYED to TAKEN--><path d="M89.37,270 C132.9,270 198.51,270 236.77,270 " fill="none" id="DELAYED-backto-TAKEN" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="84.17,270,93.17,274,89.17,270,93.17,266,84.17,270" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="42" x="139.5" y="248.0669">release</text><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="116" x="102.5" y="263.1997">(id, { delay = delay })</text><!--MD5=[6f95bcb938a0da09af97af9154a2c85e]
-link TAKEN to DONE--><path d="M264.27,295.34 C263.01,313.35 263.74,337.95 273.5,357 C280.65,370.96 293.17,382.68 305.32,391.59 " fill="none" id="TAKEN-to-DONE" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="309.42,394.51,304.4101,386.0306,305.3475,391.6092,299.7688,392.5466,309.42,394.51" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="82" x="274.5" y="345.5669">ack() / delete()</text><!--MD5=[62ca2bd2e25fd5dd593774b429fdca8e]
-link TAKEN to BURIED--><path d="M298.25,270 C317.74,270 343.16,270 364.25,270 " fill="none" id="TAKEN-to-BURIED" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="369.51,270,360.51,266,364.51,270,360.51,274,369.51,270" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="35" x="316.5" y="263.0669">bury()</text><!--MD5=[da5c5264382edc34a0d60ad59a85901f]
-link BURIED to DONE--><path d="M393.15,295.07 C381.77,319.21 364.24,356.4 352.12,382.12 " fill="none" id="BURIED-to-DONE" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="349.92,386.78,357.3764,380.3455,352.0524,382.2575,350.1404,376.9336,349.92,386.78" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="52" x="381" y="338.0669">delete() /</text><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="59" x="377.5" y="353.1997">ttl timeout</text><!--MD5=[707e10da1607e59fd17cc6f41b1726ea]
-reverse link READY to DELAYED--><path d="M258.02,138.2 C227.24,147.05 182.15,162.17 146.5,183 C117.27,200.08 88.3,225.9 68.94,244.79 " fill="none" id="READY-backto-DELAYED" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="262.83,136.83,253.0823,135.422,258.0176,138.1866,255.2529,143.1219,262.83,136.83" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="45" x="147.5" y="203.5669">timeout</text><!--MD5=[73d708825d6e7a22b23f467ee2112284]
-link DELAYED to DONE--><path d="M84.06,292.23 C117.22,310.21 166.51,336.3 210.5,357 C242.04,371.84 278.75,387.13 304.76,397.64 " fill="none" id="DELAYED-to-DONE" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="309.48,399.54,302.628,392.4654,304.8426,397.6708,299.6372,399.8853,309.48,399.54" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="44" x="211.5" y="345.5669">delete()</text><!--MD5=[ad73474698e21005f8a0eea3080e5769]
-link DONE to *end--><path d="M338.5,437.14 C338.5,459.03 338.5,490.57 338.5,508.71 " fill="none" id="DONE-to-*end" style="stroke:#A80036;stroke-width:1.0;"/><polygon fill="#A80036" points="338.5,513.88,342.5,504.88,338.5,508.88,334.5,504.88,338.5,513.88" style="stroke:#A80036;stroke-width:1.0;"/><text fill="#000000" font-family="sans-serif" font-size="13" lengthAdjust="spacingAndGlyphs" textLength="127" x="339.5" y="480.0669">remove executed task</text><!--MD5=[2b294fbeb8489447792b10e2ffdf21ce]
-@startuml
-[*]     -down->  READY   : put()
-[*]     -down->  DELAYED : put('my_task_data',\n{ delay = delay })
-READY   -down->  TAKEN   : take()
-READY   -down->  DONE    : delete() /\nttl timeout
-READY   -down->  BURIED  : bury()
-TAKEN   -up->    READY   : release() /\nttr timeout
-TAKEN   -left->  DELAYED : release\n(id, { delay = delay })
-TAKEN   -down->  DONE    : ack() / delete()
-TAKEN   -right-> BURIED  : bury()
-BURIED  -down->  DONE    : delete() /\nttl timeout
-BURIED  -up->    READY   : kick()
-DELAYED -up->    READY   : timeout
-DELAYED -down->  DONE    : delete()
-DONE    -down->  [*]     : remove executed task
-@enduml
-
-PlantUML version 1.2020.23(Sun Dec 13 19:14:58 GMT 2020)
-(GPL source distribution)
-Java Runtime: OpenJDK Runtime Environment
-JVM: OpenJDK 64-Bit Server VM
-Default Encoding: UTF-8
-Language: en
-Country: US
---></g></svg>
\ No newline at end of file
diff --git a/doc/src/statediagram.puml b/doc/src/statediagram.puml
deleted file mode 100644
index 0b554634..00000000
--- a/doc/src/statediagram.puml
+++ /dev/null
@@ -1,27 +0,0 @@
-/' This code is for generating the task state diagram in SVG format.
-Use any PlantUML rendering service, for example, https://www.planttext.com/.
-To generate the diagram on the page of this service:
-1.  In the left pane, copy and paste the code or select this source file
-    statediagram.puml with the "File Manager" button.
-2.  Click "Refresh". The resulting diagram is displayed in the right pane.
-3.  Click SVG link below the diagram view and download the diagram file.
-4.  Rename the file into "statediagram.svg" and upload in the ./doc/images/
-    folder.
-'/
-
-@startuml
-[*]     -down->  READY   : put()
-[*]     -down->  DELAYED : put('my_task_data',\n{ delay = delay })
-READY   -down->  TAKEN   : take()
-READY   -down->  DONE    : delete() /\nttl timeout
-READY   -down->  BURIED  : bury()
-TAKEN   -up->    READY   : release() /\nttr timeout
-TAKEN   -left->  DELAYED : release\n(id, { delay = delay })
-TAKEN   -down->  DONE    : ack() / delete()
-TAKEN   -right-> BURIED  : bury()
-BURIED  -down->  DONE    : delete() /\nttl timeout
-BURIED  -up->    READY   : kick()
-DELAYED -up->    READY   : timeout
-DELAYED -down->  DONE    : delete()
-DONE    -down->  [*]     : remove executed task
-@enduml