Section MPCP_PCP.
Variable R : SRS.
Variable x0 y0 : string.
Definition Sigma := sym (x0/y0 :: R).
Definition R_Sigma : sym R <<= Sigma.

Lemma MPCP_PCP x y A :
A <<= x0/y0 :: R -> x ++ tau1 A = y ++ tau2 A ->
exists B, B <<= P /\ (#_L x) ++ tau1 B = # :: #_R y ++ tau2 B.