isProperLowerBound
Given a bound of the form T <: α, we say T is a proper lower bound of α.
Return empty if it is not a proper lower bound. Otherwise it returns the variable of which this is an proper lower bound.
Given a bound of the form T <: α, we say T is a proper lower bound of α.
Return empty if it is not a proper lower bound. Otherwise it returns the variable of which this is an proper lower bound.