  Multiparty session types (MPST) provide a type discipline where a programmer
or architect specifies a whole view of communications as a global protocol, and
each distributed program is locally type-checked against its end-point
projection. After 10 years from the birth of MPST, Scalas and Yoshida
discovered that the proofs of type safety in the literature which use the
end-point projection with mergeability are flawed. After this paper,
researchers wrongly believed that the end-point projection (with mergeability)
was unsound. We correct this misunderstanding, proposing a new general proof
technique for type soundness of multiparty session $\pi$-calculus, which uses
an association relation between a global type and its end-point projection.
