error message for misplaced assume statement.

[Schaechtle]

The following bug (forgot to delete assume):

    ripl = shortcuts.make_lite_ripl()
    ripl.execute_program('''
    assume my_normal = () -> {
        assume x =  1;
        normal(0,1)

    }
    ''')
    ripl.sample('my_normal()')

Throws this uninformative error message:

../venv/local/lib/python2.7/site-packages/venture/ripl/ripl.py:897: in sample
    value = self.execute_instruction(i)['value']
../venv/local/lib/python2.7/site-packages/venture/ripl/ripl.py:274: in execute_instruction
    self._raise_annotated(e, instruction)
../venv/local/lib/python2.7/site-packages/venture/ripl/ripl.py:270: in execute_instruction
    stringable_instruction)
../venv/local/lib/python2.7/site-packages/venture/ripl/ripl.py:285: in _execute_parsed_instruction
    ans = self.sivm.execute_instruction(parsed_instruction)
../venv/local/lib/python2.7/site-packages/venture/sivm/venture_sivm.py:92: in execute_instruction
    return f(instruction)
../venv/local/lib/python2.7/site-packages/venture/sivm/venture_sivm.py:469: in _do_sample
    o1 = self._call_core_sivm_instruction(inst1)
../venv/local/lib/python2.7/site-packages/venture/sivm/venture_sivm.py:193: in _call_core_sivm_instruction
    response = self.core_sivm.execute_instruction(desugared_instruction)
../venv/local/lib/python2.7/site-packages/venture/sivm/core_sivm.py:63: in execute_instruction
    return f(instruction)
../venv/local/lib/python2.7/site-packages/venture/sivm/core_sivm.py:144: in _do_predict
    did, val = self.engine.predict(exp)
../venv/local/lib/python2.7/site-packages/venture/engine/engine.py:100: in predict
    (did, answers) = self.predict_all(datum)
../venv/local/lib/python2.7/site-packages/venture/engine/engine.py:96: in predict_all
    values = self.model.evaluate(baseAddr, datum)
../venv/local/lib/python2.7/site-packages/venture/engine/trace_set.py:152: in evaluate
    return self.traces.map('evaluate', baseAddr, datum)
../venv/local/lib/python2.7/site-packages/venture/multiprocess.py:197: in map
    for pipe in self.pipes: pipe.send((cmd, args, kwargs, None))
../venv/local/lib/python2.7/site-packages/venture/multiprocess.py:565: in send
    self.other.emplace(obj)
../venv/local/lib/python2.7/site-packages/venture/multiprocess.py:570: in emplace
    cb()
../venv/local/lib/python2.7/site-packages/venture/multiprocess.py:378: in poll
    res = [getattr(o, cmd)(*args, **kwargs) for o in self.objs]
../venv/local/lib/python2.7/site-packages/venture/multiprocess.py:351: in <lambda>
    return lambda *args, **kwargs: Success(getattr(self.obj, attrname)(*args, **kwargs))
../venv/local/lib/python2.7/site-packages/venture/engine/trace.py:62: in evaluate
    self.trace.eval(baseAddr,exp)
../venv/local/lib/python2.7/site-packages/venture/lite/trace.py:401: in eval
    Scaffold(), False, OmegaDB(), OrderedDict())
../venv/local/lib/python2.7/site-packages/venture/lite/regen.py:207: in evalFamily
    shouldRestore, omegaDB, gradients)
../venv/local/lib/python2.7/site-packages/venture/lite/regen.py:233: in apply
    shouldRestore, omegaDB, gradients)
../venv/local/lib/python2.7/site-packages/venture/lite/regen.py:298: in evalRequests
    shouldRestore, omegaDB, gradients)
../venv/local/lib/python2.7/site-packages/venture/lite/regen.py:199: in evalFamily
    shouldRestore, omegaDB, gradients)
../venv/local/lib/python2.7/site-packages/venture/lite/regen.py:180: in evalFamily
    sourceNode = env.findSymbol(exp)
../venv/local/lib/python2.7/site-packages/venture/lite/env.py:72: in findSymbol
    else: ret = self.outerEnv.findSymbol(sym)
../venv/local/lib/python2.7/site-packages/venture/lite/env.py:72: in findSymbol
    else: ret = self.outerEnv.findSymbol(sym)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <venture.lite.env.VentureEnvironment object at 0x7f0372a94ad0>, sym = 'bind_'

    def findSymbol(self,sym):
      # type: (str) -> T
      if sym in self.frame: ret = self.frame[sym]
      elif not self.outerEnv: ret = None
      else: ret = self.outerEnv.findSymbol(sym)
      if ret is None:
>       raise VentureError("Cannot find symbol '%s'" % sym)
E       VentureException: *** evaluation: Cannot find symbol 'bind_'
E       (my_normal)
E       ^^^^^^^^^^^
E       (make_csp (quote ()) (quote (bind_ (_assume (quote x) (quote 1.0)) (make_csp (quote ()) (quote (normal 0.0 1.0))))))
E                                    ^^^^^

../venv/local/lib/python2.7/site-packages/venture/lite/env.py:74: VentureException