theorem
  f = {[c,d]} implies f/.c = d
proof
  assume
A1: f = {[c,d]};
  then [c,d] in f by TARSKI:def 1;
  then
A2: c in dom f by FUNCT_1:1;
  (f qua Function).c = d by A1,GRFUNC_1:6;
  hence thesis by A2,PARTFUN1:def 6;
end;
