theorem Th28:
  a > 0 implies a to_power (-c) = 1 / a to_power c
proof
  assume
A1: a > 0;
then  a #R (-c) = 1 / a #R c by PREPOWER:76;
then  a #R (-c) = 1 / a to_power c by A1,Def2;
  hence thesis by A1,Def2;
end;
