Buggy attempt at completing SLD

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>
Fix a bug with unification

The following case was not considered:

Current substitution:

{X, Y} -> atom(args)
{Z, W} -> atom(args2)

Adding unification

X = W

needed to merge the two entries together, but previously it was assumed
adding a unification would only affect one entry of the substitution
map.

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>
Always use second argument of Assignable

Note: probably Assignable could be redone to contain only one parameter.
That parameter would just be replaced without any need to look back at
the original value.

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>
Typo in test

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>
More clear output

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>