def addingIsValid(myGrid, parentGrid):
    """
    True if adding a indices from one grid to another is considered valid.

    In ARMI we allow the addition of a 1-D axial grid with a 2-D grid.
    We do not allow any other kind of adding. This enables the 2D/1D
    grid layout in Assemblies/Blocks but does not allow 2D indexing
    in pins to become inconsistent.
    """
    return myGrid.isAxialOnly and not parentGrid.isAxialOnly