Return a set of symbols in `node` that are assigned.
    
    :param node: ast node 
    
    :returns: set of strings.

def lhs(node):
    '''
    Return a set of symbols in `node` that are assigned.
    
    :param node: ast node 
    
    :returns: set of strings.
    '''

    gen = ConditionalSymbolVisitor()
    if isinstance(node, (list, tuple)):
        gen.visit_list(node)
    else:
        gen.visit(node)
    return gen.lhs