theorem Th3: :: MSAFREE3:3

for x being set
for S being non void Signature
for Y being non-empty ManySortedSet of the carrier of S
for X being ManySortedSet of the carrier of S
for s being SortSymbol of S holds
( ( x in X . s & x in Y . s ) iff root-tree [x,s] in ((Reverse Y) "" X) . s )