null
```
Starting on goal: [think [user says [what is the text of [theory tennis]]]] and [say $x]
Goal arity 3: [think [user says [what is the text of [theory tennis]]]] and [say $x]
nextSolution
Got clause: _1 and _2
  Clause unifies to: [think [user says [what is the text of [theory tennis]]]] and [say $x]
  gdash: think [user says [what is the text of [theory tennis]]]
Goal arity 2: think [user says [what is the text of [theory tennis]]]
 Got clause: think _7
   Clause unifies to: think [user says [what is the text of [theory tennis]]]
  Clause is native.
   gdash: say $x
 Goal arity 2: say $x
  Got clause: say [it's _21 !]
    Clause unifies to: say [it's _21 !]
    gdash: user says [_22 * _23]
  Goal arity 3: user says [_22 * _23]
  Got clause: say [_53 is _54 !]
    Clause unifies to: say [_53 is _54 !]
    gdash: rewrite with theory "drop question mark from user input"
  Goal arity 4: rewrite with theory "drop question mark from user input"
   Got clause: rewrite with theory _86
     Clause unifies to: rewrite with theory "drop question mark from user input"
    Clause is native.
Starting on goal: user says _88
Goal arity 3: user says _88
Got clause: user says [what is the text of [theory tennis]]
  Clause unifies to: user says [what is the text of [theory tennis]]
  gdash: _87 = drop question mark ([what is the text of [theory tennis]])
Goal arity 6: _87 = drop question mark ([what is the text of [theory tennis]])
 Got clause: _123 = drop question mark (_124)
   Clause unifies to: _87 = drop question mark ([what is the text of [theory tennis]])
   gdash: _125 = tocons ([what is the text of [theory tennis]])
 Goal arity 4: _125 = tocons ([what is the text of [theory tennis]])
  Got clause: _142 = tocons (_143)
    Clause unifies to: _125 = tocons ([what is the text of [theory tennis]])
   Clause is native.
    gdash: endswithword_c [cons what [cons is [cons the [cons text [cons of [cons [theory tennis] end]]]]]] ?
  Goal arity 3: endswithword_c [cons what [cons is [cons the [cons text [cons of [cons [theory tennis] end]]]]]] ?
   Got clause: endswithword_c [cons _147 _148] _149
     Clause unifies to: endswithword_c [cons what [cons is [cons the [cons text [cons of [cons [theory tennis] end]]]]]] ?
     gdash: endswithword_c [cons is [cons the [cons text [cons of [cons [theory tennis] end]]]]] ?
   Goal arity 3: endswithword_c [cons is [cons the [cons text [cons of [cons [theory tennis] end]]]]] ?
    Got clause: endswithword_c [cons _153 _154] _155
      Clause unifies to: endswithword_c [cons is [cons the [cons text [cons of [cons [theory tennis] end]]]]] ?
      gdash: endswithword_c [cons the [cons text [cons of [cons [theory tennis] end]]]] ?
    Goal arity 3: endswithword_c [cons the [cons text [cons of [cons [theory tennis] end]]]] ?
     Got clause: endswithword_c [cons _159 _160] _161
       Clause unifies to: endswithword_c [cons the [cons text [cons of [cons [theory tennis] end]]]] ?
       gdash: endswithword_c [cons text [cons of [cons [theory tennis] end]]] ?
     Goal arity 3: endswithword_c [cons text [cons of [cons [theory tennis] end]]] ?
      Got clause: endswithword_c [cons _165 _166] _167
        Clause unifies to: endswithword_c [cons text [cons of [cons [theory tennis] end]]] ?
        gdash: endswithword_c [cons of [cons [theory tennis] end]] ?
      Goal arity 3: endswithword_c [cons of [cons [theory tennis] end]] ?
       Got clause: endswithword_c [cons _171 _172] _173
         Clause unifies to: endswithword_c [cons of [cons [theory tennis] end]] ?
         gdash: endswithword_c [cons [theory tennis] end] ?
       Goal arity 3: endswithword_c [cons [theory tennis] end] ?
        Got clause: endswithword_c [cons _177 _178] _179
          Clause unifies to: endswithword_c [cons [theory tennis] end] ?
          gdash: endswithword_c end ?
        Goal arity 3: endswithword_c end ?
Exception in native class main$RewriteWithTheory: java.lang.NullPointerException
	at main$Prolog.Trail_Undo(main.java:167)
	at main$Prolog.rewriteWith(main.java:943)
	at main$RewriteWithTheory.yo(main.java:1422)
	at main$Prolog$BaseNative.yo(main.java:841)
	at main$Prolog.step(main.java:561)
	at main$Prolog.nextSolution(main.java:682)
	at main$Prolog.solve(main.java:657)
	at main$Prolog.solve(main.java:661)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at main.call(main.java:861)
	at main.queryAI(main.java:184)
	at main.answer(main.java:135)
	at sun.reflect.GeneratedMethodAccessor2251.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at main.callOpt(main.java:543)
	at main.callStaticAnswerMethod(main.java:842)
	at main.answer(main.java:438)
	at sun.reflect.GeneratedMethodAccessor3594.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at main.callOpt(main.java:1896)
	at main.callStaticAnswerMethod(main.java:2193)
	at main.callDehBots(main.java:740)
	at main.answerImpl(main.java:729)
	at main.answer(main.java:781)
	at main$Channel.tendTo(main.java:131)
	at main.slackBotLoop(main.java:312)
	at main.main(main.java:948)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at x30.callOpt(x30.java:4547)
	at x30.callMain(x30.java:3405)
	at x30.runMainMethod(x30.java:1509)
	at x30.runProgramQuick(x30.java:1488)
	at x30.runProgram(x30.java:1428)
	at x30.javax2(x30.java:1155)
	at x30.javaxmain(x30.java:269)
	at x30.goMain(x30.java:195)
	at x30.main(x30.java:82)

Native clause fails

No solution```