Issue #4 resolved

Cannot use unary minus on a variable when evaluating arithmetic.

Edd Barrett
created an issue

Under SWI Prolog, this is a valid query:

?- X = 1, Y is -X.
X = 1,
Y = -1.

Under pyrolog, we see:

>?-  X = 1, Y is -X.
ERROR: Type error: 'evaluable' expected, found '-/1'

Noe that unary minus works for constants in pyrolog:

>?- X is -1.
yes
X = -1