theorem Th123: :: SHEFFER2:123

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