Quasirationality and prounipotent crossed modules

Quasirational presentations ($QR$-presentations) of (pro-$p$)groups are studied. Such presentations include, in particular, aspherical presentations of discrete groups and their subpresentations and pro-$p$-groups with a single defining relation. Using schematization of $QR$-presentations we answer a variation of J.-P. Serre's question on one-relator pro-$p$-groups.
Comment: Text structure has been reorganized. Material on Melnikov's conjecture was singled out into a different article. Now in English, still a preliminary version any comments are wellcome