proposition: Proposition Book_1_2_sig_fst : @pr1 A B = sig_rec_proj A (fun a => fun b => a).
proof: Proof. reflexivity. Defined.
imports: HoTT(Basics Types HProp HSet Projective TruncType Truncations Modalities.Notnot Modalities.Open Modalities.Closed BoundedSearch Equiv.BiInv Spaces.Nat Spaces.Torus.TorusEquivCircles Metatheory.Core Metatheory.FunextVarieties)