continuum: make "monomials" preserve order of operators

Probably there should be a flag that, if set, preserves operator order. This would only be used if the input expression is a string, as when the input is a sympy expression, the constituent symbols will already have any commutation relations defined

monomials is only accepting sympy expression as its input

shouldn't we then rely on the commutativity of the sympy symbols to tell us what expressions are equal, and which are not?

In the context of the Landau level project, this would mean that there would need to be momentum operators defined as non-commutative.

Depending on the symbols commutative relation is the main point of this issue ; - ).

What do you mean by "what expressions are equal"? Why do we need it here?

Just to make a note here: it seems like a good option to specify in the docstring that we preserve order of generators only between them, not between other symbols in input expression.

For example (new behaviour):

>>> x, y, A = sympy.symbols('x y A', commutative=False)
>>> expr = x**2 * A * y + y * x**2
>>> monomials(expr, x, y)
{x**2 * y: A, y * x**2: 1}

once old would give

{x**2 * y: A + 1}

changed milestone to %Kwant 1.3.2

added difficulty::medium type:enhancement priority::normal labels

removed milestone

mentioned in merge request !151 (merged)

closed via commit e7fa76a2

closed via commit eb66bfe8

mentioned in commit e7fa76a2