let S be non empty non void ManySortedSign ; :: thesis: for U0 being non-empty MSAlgebra of S
for U1, U2 being strict MSSubAlgebra of U0 holds U1 /\ (U1 "\/" U2) = U1
let U0 be non-empty MSAlgebra of S; :: thesis: for U1, U2 being strict MSSubAlgebra of U0 holds U1 /\ (U1 "\/" U2) = U1
let U1, U2 be strict MSSubAlgebra of U0; :: thesis: U1 /\ (U1 "\/" U2) = U1
reconsider u1 = the Sorts of U1, u2 = the Sorts of U2 as MSSubset of U0 by Def10;
A1: the Sorts of (U1 /\ (U1 "\/" U2)) = the Sorts of U1 /\ the Sorts of (U1 "\/" U2) by Def17;
( u1 c= the Sorts of U0 & u2 c= the Sorts of U0 ) by PBOOLE:def 18;
then u1 \/ u2 c= the Sorts of U0 by PBOOLE:16;
then reconsider A = u1 \/ u2 as MSSubset of U0 by PBOOLE:def 18;
U1 "\/" U2 = GenMSAlg A by Def19;
then A is MSSubset of (U1 "\/" U2) by Def18;
then A2: A c= the Sorts of (U1 "\/" U2) by PBOOLE:def 18;
the Sorts of U1 c= A by PBOOLE:14;
then the Sorts of U1 c= the Sorts of (U1 "\/" U2) by A2, PBOOLE:13;
then A3: the Sorts of U1 c= the Sorts of (U1 /\ (U1 "\/" U2)) by A1, PBOOLE:17;
reconsider u112 = the Sorts of (U1 /\ (U1 "\/" U2)) as MSSubset of U0 by Def10;
A4: the Charact of (U1 /\ (U1 "\/" U2)) = Opers (U0,u112) by Def17;
the Sorts of (U1 /\ (U1 "\/" U2)) c= the Sorts of U1 by A1, PBOOLE:15;
then the Sorts of (U1 /\ (U1 "\/" U2)) = the Sorts of U1 by A3, PBOOLE:def 10;
hence U1 /\ (U1 "\/" U2) = U1 by A4, Def10; :: thesis: verum