def is_one2one(self):
return is_one2one(self.fmt)<EOL>
Runs a check to evaluate if this format string has a
one to one correspondence.  I.e. that successive composing and
parsing opperations will result in the original data.
In other words, that input data maps to a string,
which then maps back to the original data without any change
or loss in information.

Note: This test only applies to sensible usage of the format string.
If string or numeric data is causes overflow, e.g. 
if composing "abcd" into {3s}, one to one correspondence will always 
be broken in such cases. This off course also applies to precision 
losses when using  datetime data.