using comma in argument to sample fetch & converter

[Patrick Hemmer]

How do you use a comma inside an argument to a sample fetcher or converter?
For example, the sample fetch str, if I try to do `str(foo,bar)` I get
the error

    fetch method 'str' : end of arguments expected at position 2, but
got ',bar'

All variations such as `str('foo,bar')`, `str(foo\,bar)`, etc, result in
the same error.

-Patrick