Consume chars in set from the string and return (subtxt,offset).

    Example:

        text = "foo(bar)"
        chars = set('abcdefghijklmnopqrstuvwxyz')

        name,off = nom(text,0,chars)

def nom(txt, off, cset, trim=True):
    '''
    Consume chars in set from the string and return (subtxt,offset).

    Example:

        text = "foo(bar)"
        chars = set('abcdefghijklmnopqrstuvwxyz')

        name,off = nom(text,0,chars)

    '''
    if trim:
        while len(txt) > off and txt[off] in whites:
            off += 1

    r = ''
    while len(txt) > off and txt[off] in cset:
        r += txt[off]
        off += 1

    if trim:
        while len(txt) > off and txt[off] in whites:
            off += 1

    return r, off