def is_indvar(expr):
 assert isinstance(expr, string_types), (u'%s is not a string' % expr)
 return (re.match(u'^[a-df-z]\\d*$', expr) is not None)

def is_indvar ( expr ) : assert isinstance ( expr , string_types ) , ( u'%s is not a string' % expr ) return ( re . match ( u'^[a-df-z]\\d*$' , expr ) is not None )
an individual variable must be a single lowercase character other than e .