Cypher IF A AND NOT(B) in match

I want to write a cypher clause that says (I know this is not correct syntax, its just an example to show the general idea):

IF (S1)-->(B1{attr:TRUE})-->(G) AND NOT((S2)-->(B2{attr:FALSE})-->(G))
THEN stuff

So, if I have only the node B1 with attr=TRUE I want the pattern to match if I have B1 with attr=TRUE and also B2 with attr=FALSE I want the pattern not to match. In all other cases where at least B1 with attr=TRUE is found, the mattern should match also.

But I cannot figure out how to implement this logic.