theorem
  c in f"SD iff [c,f/.c] in f & f/.c in SD
proof
  thus c in f"SD implies [c,f/.c] in f & f/.c in SD
  proof
    assume
A1: c in f"SD;
    then
A2: (f qua Function).c in SD by GRFUNC_1:26;
A3: [c,(f qua Function).c] in f by A1,GRFUNC_1:26;
    then c in dom f by FUNCT_1:1;
    hence thesis by A3,A2,PARTFUN1:def 6;
  end;
  assume that
A4: [c,f/.c] in f and
A5: f/.c in SD;
  c in dom f by A4,Th46;
  hence thesis by A5,Th26;
end;
