Another Binary Relation Implementation
lWith each first element of some pair, give the list of second elements to which the first element is related:
l[[ÒAÓ,   [ÒAÓ, ÒABÓ]],
 [ÒBÓ,    [ÒBÓ, ÒABÓ],
 [ÒABÓ, [ÒABÓ]],
 [ÒOÓ,   [ÒOÓ, ÒAÓ, ÒBÓ, ÒABÓ]]

lCompare the two representations.