Parses the sole alphabetic character value with name tag_name in tag t. Heavy-handed with the asserts.

def parse_singular_alphabetic_character(t, tag_name):
    '''Parses the sole alphabetic character value with name tag_name in tag t. Heavy-handed with the asserts.'''
    pos = t.getElementsByTagName(tag_name)
    assert(len(pos) == 1)
    pos = pos[0]
    assert(len(pos.childNodes) == 1)
    v = pos.childNodes[0].data
    assert(len(v) == 1 and v >= 'A' and 'v' <= 'z') # no floats allowed
    return v