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: java.lang.NullPointerException
Native clause fails

No solution```