scheme :: RELSET_1:sch 1
RelOnSetEx{ F₁() -> set , F₂() -> set , P₁[ object , object ] } :

ex R being Relation of F₁(),F₂() st
for x, y being object holds
( [x,y] in R iff ( x in F₁() & y in F₂() & P₁[x,y] ) )