be an exact integer, either 2, 8,
     10, or 16.  If supplied, RADIX is a default radix that may be
     overridden by an explicit radix prefix in STRING (e.g.  "#o177").
     If RADIX is not supplied, then the default radix is 10.  If string
     is not a syntactically valid notation for a number, then
     `string->number' returns `#f'.

