reserve A,B,C for Ordinal;
reserve a,b,c,d for natural Ordinal;
reserve l,m,n for natural Ordinal;
reserve i,j,k for Element of omega;
reserve x,y,z for Element of RAT+;
reserve i,j,k for natural Ordinal;
reserve r,s,t for Element of RAT+;

theorem Th86:
  r <=' s & s <=' r + t implies ex t0 being Element of RAT+ st s =
  r + t0 & t0 <=' t
by Th76;
