The representation '*' was shared with NodeWildcard. I believe '*'
is better for NodeWildcard so let's change Gap to '~'.
At first I wanted to use '_' but it is already occupied by
BottomOfTheStackSymbol and the underscore is probably better suited for
that.

inlining prevents extern from working with these functions

even though the count result casted to boolean is semantically the same, contains is more expressive