theorem
  c1 in dom f & c2 in dom f implies f.:{c1,c2} = {f/.c1,f/.c2}
proof
  assume that
A1: c1 in dom f and
A2: c2 in dom f;
  thus f.:{c1,c2} = {(f qua Function).c1,(f qua Function).c2} by A1,A2,
FUNCT_1:60
    .= {f/.c1,(f qua Function).c2} by A1,PARTFUN1:def 6
    .= {f/.c1,f/.c2} by A2,PARTFUN1:def 6;
end;
