fact: Hypothesis H2 : Rabs (y-ty) + Rabs (a*x-ta*tx) <= bpow (-prec-2)*(1-bpow (1-prec))*Rabs ty - bpow (-prec-2)*Rabs (ta*tx) - bpow (emin-2).
imports: Psatz Core Plus_error Mult_error Operations Sterbenz Pff Pff2FlocqAux Nat2Z_compat