def _allocate_expr_id_instance(name, exprmap):
    """ Allocate a new free instance of a name

    Args:
        name:     Existing name
        exprmap:  Map of existing expression names
    Returns:
        New id not in exprmap
    """
    ndx = 1
    name += "@"
    nname = name + "1"
    while nname in exprmap:
        ndx += 1
        nname = name + str(ndx)
    return nname