FUNCTION gammp(a,x: real): real;
VAR
   gammcf,gln: real;
BEGIN
   IF ((x < 0.0) OR (a <= 0.0)) THEN BEGIN
      writeln('pause in GAMMP - invalid arguments'); readln
   END;
   IF (x < (a+1.0)) THEN BEGIN
      gser(a,x,gammcf,gln);
      gammp := gammcf
   END ELSE BEGIN
      gcf(a,x,gammcf,gln);
      gammp := 1.0-gammcf
   END
END;