theorem Th129: :: SHEFFER2:129

for L being non empty satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3 ShefferStr
for x, z, y being Element of L holds (((y | y) | z) | (x | z)) | (((y | y) | z) | (x | z)) = ((x | x) | ((y | y) | z)) | z