Parse table column properties.

def parse_table_column_properties(doc, cell, prop):
    "Parse table column properties."

    if not cell:
        return

    grid = prop.find(_name('{{{w}}}gridSpan'))

    if grid is not None:
        cell.grid_span = int(grid.attrib[_name('{{{w}}}val')])

    vmerge = prop.find(_name('{{{w}}}vMerge'))

    if vmerge is not None:
        if _name('{{{w}}}val') in vmerge.attrib:
            cell.vmerge = vmerge.attrib[_name('{{{w}}}val')]
        else:
            cell.vmerge = ""