isAnInstantiation
Given a bound of the form α = T or T = α, we say T is an instantiation of α.
Return empty if it is not an instantiation. Otherwise it returns the variable of which this is an instantiation.
Given a bound of the form α = T or T = α, we say T is an instantiation of α.
Return empty if it is not an instantiation. Otherwise it returns the variable of which this is an instantiation.