combine "query" and "given" statements

[till-tietz]

Problem: Currently queries of the form: (y given x = 1) - (y given x = 0) cannot be requested.

Reason: currently only a single "given" can be specified for each query (but queries of the above form require two givens i.e. x = 1 & x = 0)

Potential Solution: integrate "given" into "query" statement such that the above is expressed as:
either: Y[X = 1 | A == 0] - Y[X = 1 | A == 0]
or: Y[X = 1] - Y[X = 0] | X==1 & Y==1

Parsing needs to be updated to handle integrated queries