NotImplementedError in lambda DRT simplification.

[RnDevelover]

The following piece of code gives NotImplementedError on line 264 of drt.py (3.6.4)

import nltk
expr="\P.\Q.\R.(([X], []) + R(X) + ((P(\Y.([],[ (X=Y) ]))) + (Q(\Z.([], [ (X=Z) ])))))(\X.(\P.(([X],[named(X, karla)]) + P(X) )(\X1.(X(X1)))))(\Y.(\P.\Q. (([X],[ant(X)]) + P(X) + Q(X) )(\Y1.(\X.([],[owner(X) ] )(Y1)))(\Z1.(Y(Z1)))))(\Z.(\P.\X.\F.(P(\E.(([],[nsubj(E,X)]) + F(E))))(\X2.(\F.(([E][arrive(E)]) + F(E) )(\Z2.(X2(Z2)))))(Z)(\Y2.(\F.( [],[] )(Y2)))))"
dexpr = nltk.sem.drt.DrtExpression.fromstring(r"{}".format(expr))
dexpr.simplify()
print(str(dexpr))

When I track the cause, I find that it is happening when get_refs is called in DrtLambdaExpression. Since it does not exist, the get_refs function of the abstract class is called (line 264) giving a not implemented error.

[RnDevelover]

I made a pull request that fixes this issue.