How to use the kiwisolver.Solver function in kiwisolver

if before:
            if before not in variables:
                msg = "item '%s' has invalid `before` reference '%s'"
                raise ValueError(msg % (node.path, before))
            target_var = variables[before]
            constraints.append((this_var + 0.1 <= target_var) | 'strong')
        after = node.item.after
        if after:
            if after not in variables:
                msg = "item '%s' has invalid `after` reference '%s'"
                raise ValueError(msg % (node.path, after))
            target_var = variables[after]
            constraints.append((target_var + 0.1 <= this_var) | 'strong')
        prev_var = this_var

    solver = kiwi.Solver()
    for cn in constraints:
        solver.addConstraint(cn)
    solver.updateVariables()

    return sorted(nodes, key=lambda node: (variables[node.id].value(), id(node)))

def __init__(self):
        self._solver = kiwi.Solver()
        self._edit_stack = []
        self._initialized = False
        self._running = False

"""
        raise NotImplementedError


class LayoutManager(Atom):
    """ A class which manages the layout for a system of items.

    This class is used by the various in-process backends to simplify
    the task of implementing constraint layout management.

    """
    #: The primary layout item which owns the layout.
    _root_item = Typed(LayoutItem)

    #: The solver used by the layout manager.
    _solver = Typed(kiwi.Solver, ())

    #: The stack of edit variables added to the solver.
    _edit_stack = List()

    #: The list of layout items handled by the manager.
    _layout_items = List()

    def __init__(self, item):
        """ Initialize a LayoutManager.

        Parameters
        ----------
        item : LayoutItem
            The layout item which contains the widget which is the
            root of the layout system. This item is the conceptual
            owner of the system. It is not resized by the manager,