fact: Variable Pcompose : forall s d d' m1 m2, @Pmor d d' m1 -> @Pmor s d m2 -> @Pmor s d' (m1 o m2).
imports: HoTT.Tactics Types.Forall Types.Sigma Basics.Trunc Category.Core Functor.Core Category.Sigma.Core Functor.Paths