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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _process_constraint_expressions(self, expr_list, conflict_check=True, rev=True):
 """Create and return constraints from expressions in expr_list. :param expr_list: The expressions :conflict_check: If True, check for conflicting expressions within each field :return: Constraints grouped by field (the key is the field name) :rtype: dict """

        # process expressions, grouping by field
        groups = {}
        for expr in expr_list:
            field, raw_op, val = parse_expr(expr)
            op = ConstraintOperator(raw_op)
            if field not in groups:
                groups[field] = ConstraintGroup(Field(field, self._aliases))
            groups[field].add_constraint(op, val)

        # add existence constraints
        for cgroup in groups.values():
            cgroup.add_existence(rev)

        # optionally check for conflicts
        if conflict_check:
            # check for conflicts in each group
            for field_name, group in groups.items():
                conflicts = group.get_conflicts()
                if conflicts:
                    raise ValueError('Conflicts for field {}: {}'.format(field_name, conflicts))
        return groups