R4RS 6.5.6  (string->number string)          ==>  integer
            (string->number string integer)  ==>  integer

Returns a number of the maximally precise representation expressed
by the given STRING. INTEGER must be an exact integer, either 2, 8,
10, or 16. If supplied, INTEGER is a default radix that may be
overridden by an explicit radix prefix in string (e.g. "#o177").
If INTEGER 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.

(string->number "100")     ==>  100
(string->number "100" 16)  ==>  256
(string->number "1e2")     ==>  100.0
(string->number "1.23")    ==>  1.23
(string->number "#i10")    ==>  10.0

The S9fES implementation supports base prefixes. It also supports
decimal points, exponent markers, unspecific digits (#), and
exact/inexact prefixes, if real number support is compiled in.
