let Y be set ; :: thesis: for a, b being Element of (EqRelLatt Y)
for A, B being Equivalence_Relation of Y st a = A & b = B holds
( a [= b iff A c= B )
let a, b be Element of (EqRelLatt Y); :: thesis: for A, B being Equivalence_Relation of Y st a = A & b = B holds
( a [= b iff A c= B )
let A, B be Equivalence_Relation of Y; :: thesis: ( a = A & b = B implies ( a [= b iff A c= B ) )
assume that
A1: a = A and
A2: b = B ; :: thesis: ( a [= b iff A c= B )
thus ( a [= b implies A c= B ) :: thesis: ( A c= B implies a [= b )

proof

assume A3: a [= b ; :: thesis: A c= B
A /\ B = the L_meet of (EqRelLatt Y) . (A,B) by MSUALG_5:def 2
.= a "/\" b by A1, A2, LATTICES:def 2
.= A by A1, A3, LATTICES:4 ;
hence A c= B by XBOOLE_1:17; :: thesis: verum

end;

thus ( A c= B implies a [= b ) :: thesis: verum

proof

assume A4: A c= B ; :: thesis: a [= b
a "/\" b = the L_meet of (EqRelLatt Y) . (A,B) by A1, A2, LATTICES:def 2
.= A /\ B by MSUALG_5:def 2
.= a by A1, A4, XBOOLE_1:28 ;
hence a [= b by LATTICES:4; :: thesis: verum

end;