:: deftheorem Def3 defines MidSp-like MIDSP_1:def 3 :
for IT being non empty MidStr holds
( IT is MidSp-like iff for a, b, c, d being Element of IT holds
( a @ a = a & a @ b = b @ a & (a @ b) @ (c @ d) = (a @ c) @ (b @ d) & ex x being Element of IT st x @ a = b ) );