def _remzero(coef,fun): 
	"""coef, fun represents an expression.
	For example, if expression=5f(2,3)+0f(4,6)+8f(1,4)
	then coef=[5,0,8], fun=[[2,3],[4,6],[1,4]]
	_remzero removes addends with coefficient equal to zero.
	In this example ncoef=[5,8], nfun=[[2,3],[1,4]]"""
	ncoef=[]
	nfun=[]
	for c,f in zip(coef,fun):
		if c>0:
			ncoef+=[c]
			nfun+=[f]
	return ncoef,nfun