theorem Th90: :: SHEFFER2:90

for L being non empty satisfying_Sheffer_1 satisfying_Sheffer_2 satisfying_Sheffer_3 ShefferStr
for q, p being Element of L holds (p | p) | (p | ((q | q) | q)) = (((q | q) | (q | q)) | p) | ((q | q) | p)