merge (and probably transform) fail if same expression is used twice for creating different columns

[code-of-kpp]

Sample code:

x = blaze.Data(...)  # tested with pandas.DataFrame and mysql
blaze.merge(
    a=x.time - x.time % 3,
    b=x.time % 3,
)

Error:

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
/tmp/python3.4/site-packages/IPython/core/formatters.py in __call__(self, obj)
    695                 type_pprinters=self.type_printers,
    696                 deferred_pprinters=self.deferred_printers)
--> 697             printer.pretty(obj)
    698             printer.flush()
    699             return stream.getvalue()

/tmp/python3.4/site-packages/IPython/lib/pretty.py in pretty(self, obj)
    381                             if callable(meth):
    382                                 return meth(obj, self, cycle)
--> 383             return _default_pprint(obj, self, cycle)
    384         finally:
    385             self.end_group()

/tmp/python3.4/site-packages/IPython/lib/pretty.py in _default_pprint(obj, p, cycle)
    501     if _safe_getattr(klass, '__repr__', None) not in _baseclass_reprs:
    502         # A user-provided repr. Find newlines and replace them with p.break_()
--> 503         _repr_pprint(obj, p, cycle)
    504         return
    505     p.begin_group(1, '<')

/tmp/python3.4/site-packages/IPython/lib/pretty.py in _repr_pprint(obj, p, cycle)
    683     """A pprint that just redirects to the normal repr function."""
    684     # Find newlines and replace them with p.break_()
--> 685     output = repr(obj)
    686     for idx,output_line in enumerate(output.splitlines()):
    687         if idx:

/tmp/python3.4/site-packages/blaze/interactive.py in expr_repr(expr, n)
    259     if (ndim(expr) == 1 and (istabular(expr.dshape) or
    260                              isscalar(expr.dshape.measure))):
--> 261         return repr_tables(expr, 10)
    262 
    263     # Smallish arrays

/tmp/python3.4/site-packages/blaze/interactive.py in repr_tables(expr, n)
    198 
    199 def repr_tables(expr, n=10):
--> 200     result = concrete_head(expr, n).rename(columns={None: ''})
    201 
    202     if isinstance(result, (DataFrame, Series)):

/tmp/python3.4/site-packages/blaze/interactive.py in concrete_head(expr, n)
    180         return odo(head, object)
    181     elif isrecord(expr.dshape.measure):
--> 182         return odo(head, DataFrame)
    183     else:
    184         df = odo(head, DataFrame)

/tmp/python3.4/site-packages/odo/odo.py in odo(source, target, **kwargs)
     88     odo.append.append      - Add things onto existing things
     89     """
---> 90     return into(target, source, **kwargs)

/tmp/python3.4/site-packages/multipledispatch/dispatcher.py in __call__(self, *args, **kwargs)
    162             self._cache[types] = func
    163         try:
--> 164             return func(*args, **kwargs)
    165 
    166         except MDNotImplementedError:

/tmp/python3.4/site-packages/blaze/interactive.py in into(a, b, **kwargs)
    303 @dispatch((object, type, str, unicode), Expr)
    304 def into(a, b, **kwargs):
--> 305     result = compute(b, **kwargs)
    306     kwargs['dshape'] = b.dshape
    307     return into(a, result, **kwargs)

/tmp/python3.4/site-packages/multipledispatch/dispatcher.py in __call__(self, *args, **kwargs)
    162             self._cache[types] = func
    163         try:
--> 164             return func(*args, **kwargs)
    165 
    166         except MDNotImplementedError:

/tmp/python3.4/site-packages/blaze/interactive.py in compute(expr, **kwargs)
    165         raise ValueError("No data resources found")
    166     else:
--> 167         return compute(expr, resources, **kwargs)
    168 
    169 

/tmp/python3.4/site-packages/multipledispatch/dispatcher.py in __call__(self, *args, **kwargs)
    162             self._cache[types] = func
    163         try:
--> 164             return func(*args, **kwargs)
    165 
    166         except MDNotImplementedError:

/tmp/python3.4/site-packages/blaze/compute/core.py in compute(expr, d, **kwargs)
    470         d4 = d3
    471 
--> 472     result = top_then_bottom_then_top_again_etc(expr3, d4, **kwargs)
    473     if post_compute_:
    474         result = post_compute_(expr3, result, scope=d4)

/tmp/python3.4/site-packages/blaze/compute/core.py in top_then_bottom_then_top_again_etc(expr, scope, **kwargs)
    175     if optimize_:
    176         try:
--> 177             expr3 = optimize_(expr2, *[scope3[leaf] for leaf in expr2._leaves()])
    178             _d = dict(zip(expr2._leaves(), expr3._leaves()))
    179             scope4 = dict((e._subs(_d), d) for e, d in scope3.items())

/tmp/python3.4/site-packages/blaze/compute/core.py in <listcomp>(.0)
    175     if optimize_:
    176         try:
--> 177             expr3 = optimize_(expr2, *[scope3[leaf] for leaf in expr2._leaves()])
    178             _d = dict(zip(expr2._leaves(), expr3._leaves()))
    179             scope4 = dict((e._subs(_d), d) for e, d in scope3.items())

KeyError: _5