Fact MPCP_SR x y A :
A <<= P -> x <<= Sigma -> y <<= Sigma ->
tau1 A = x ++ [#] ++ y ++ tau2 A ->
y ++ x ≻* y0.

Corollary SR_MPCP_cor :
x0 ≻* y0 <-> exists A, A <<= P /\ tau1 (d :: A) = tau2 (d :: A).