# Source code for ufl.algorithms.traversal

    """This module contains algorithms for traversing expression trees in different ways."""

    # Copyright (C) 2008-2016 Martin Sandve Alnæs and Anders Logg
    #
    # This file is part of UFL (https://www.fenicsproject.org)
    #
    # SPDX-License-Identifier:    LGPL-3.0-or-later
    #
    # Modified by Anders Logg, 2008

    from ufl.action import Action
    from ufl.adjoint import Adjoint
    from ufl.core.expr import Expr
    from ufl.form import BaseForm, Form, FormSum
    from ufl.integral import Integral

    def iter_expressions(a):
        """Handle Form, Integral and any Expr the same way when inspecting expressions.

        Returns an iterable over Expr instances:
        - a is an Expr: (a,)
        - a is an Integral:  the integrand expression of a
        - a is a  Form:      all integrand expressions of all integrals
        - a is a  FormSum:   the components of a
        - a is an Action:    the left and right component of a
        - a is an Adjoint:   the underlying form of a
        """
        if isinstance(a, Form):
            return (itg.integrand() for itg in a.integrals())
        elif isinstance(a, Integral):
            return (a.integrand(),)
        elif isinstance(a, FormSum | Adjoint | Action):
            return tuple(e for op in a.ufl_operands for e in iter_expressions(op))
        elif isinstance(a, Expr | BaseForm):
            return (a,)
        raise ValueError(f"Not an UFL type: {type(a)}")