reserve L for D_Lattice;
reserve a, b, c for Element of L;

theorem Th10:
  c"/\"a = c"/\"b & c"\/"a = c"\/"b implies a=b
proof
  assume that
A1: c"/\"a = c"/\"b and
A2: c"\/"a = c"\/"b;
  thus a = a"/\"( c"\/"a ) by Def9
    .= ( b"/\"c )"\/"( b"/\"a ) by A1,A2,Def11
    .= b"/\"( c"\/"a ) by Def11
    .= b by A2,Def9;
end;
