def singularize(args, premise):
    """Return singular equivalent POS-tag for given POS-tag."""  

    pos = args[0]
    
    # Singularize dict
    singularize_dict = {'NNS':'NN',
                      'NN':'NN',
                      'NNPS':'NNP',
                      'VBP':'VBZ',
                      'VBD':'VBD',
                      'VB':'VB',
                      'VBZ':'VBZ',
                      'VBN':'VBN',
                      'MD':'MD'   }
    
    # Test whether pos is in dict, otherwise return unaltered pos
    try:
        sing_pos = singularize_dict[pos]
    except KeyError:
        return pos
    return sing_pos