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

Lemma match_start d' B :
d' :: B <<= P -> tau1 (d' :: B) = tau2 (d' :: B) -> d' = d.