REPEAT
        READ N$
        IF FN_isanumber(N$) THEN
          PRINT "'" N$ "' is a number"
        ELSE
          PRINT "'" N$ "' is NOT a number"
        ENDIF
      UNTIL N$ = "end"
      END

      DATA "PI", "0123", "-0123", "12.30", "-12.30", "123!", "0"
      DATA "0.0", ".123", "-.123", "12E3", "12E-3", "12+3", "end"

      DEF FN_isanumber(A$)
      ON ERROR LOCAL = FALSE
      IF EVAL("(" + A$ + ")") <> VAL(A$) THEN = FALSE
      IF VAL(A$) <> 0 THEN = TRUE
      IF LEFT$(A$,1) = "0" THEN = TRUE
      = FALSE