def _get_const_index_expr_inner(stencil_ir, func_ir, index_var):
    """inner constant inference function that calls constant, unary and binary
    cases.
    """
    require(isinstance(index_var, ir.Var))
    # case where the index is a const itself in outer function
    var_const =  guard(_get_const_two_irs, stencil_ir, func_ir, index_var)
    if var_const is not None:
        return var_const
    # get index definition
    index_def = ir_utils.get_definition(stencil_ir, index_var)
    # match inner_var = unary(index_var)
    var_const = guard(
        _get_const_unary_expr, stencil_ir, func_ir, index_def)
    if var_const is not None:
        return var_const
    # match inner_var = arg1 + arg2
    var_const = guard(
        _get_const_binary_expr, stencil_ir, func_ir, index_def)
    if var_const is not None:
        return var_const
    raise GuardException
inner constant inference function that calls constant, unary and binary
    cases.
    
numba/stencils/stencilparfor.py