def merge_maps(m, base):
    """
    Merge in undefined map entries from given map.
    
    @param m: Map to be merged into.
    @type m: lems.util.Map
    
    @param base: Map to be merged into.
    @type base: lems.util.Map
    """
    
    for k in base.keys():
        if k not in m:
            m[k] = base[k]