Return a string holding the external representation of the
number n in the given radix. If n is inexact, a radix of 10 will
be used. Return a number of the maximally precise representation
expressed by the given string. radix must 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->numberabove, but taking a C string, as
pointer and length. The string characters should be in the
current locale encoding ( localein the name refers only to that,
there's no locale-dependent parsing).