: The first form steps through the sequence
identifying characters that are part of a format sequence.
Each character that is not part of a format sequence
is written to s
each format sequence, as it is identified, results in a call to do_put
thus, format elements and other characters are interleaved in the output
in the order in which they appear in the pattern.
Format sequences are identified by converting each character c
value as if by ct.narrow(c, 0)
is a reference to ctype<charT>
obtained from str.getloc().
The first character of each sequence is equal to '%'
followed by an optional modifier character mod
and a format specifier character spec
as defined for the function strftime.
If no modifier character is present, mod
For each valid format sequence identified,
calls do_put(s, str, fill, t, spec, mod).