word(0) :- write('zero').
word(1) :- write('one').
word(2) :- write('two').
word(3) :- write('three').
word(4) :- write('four').
word(5) :- write('five').
word(6) :- write('six').
word(7) :- write('seven').
word(8) :- write('eight').
word(9) :- write('nine').
 
num_words(Nums) :-       % This is top-level predicate.
    NDiv is Nums // 10,  % It prints the first digit unconditionally,
    dash_num_words(NDiv),% letting you handle the case when the number is zero.
    NMod is Nums mod 10, 
    word(NMod).
 
dash_num_words(0).       % When we reach zero, we stop printing.
 
dash_num_words(Nums) :-  % Otherwise, we follow your algorithm
    Nums > 0,            % with one modification: the dash is printed
    NDiv is Nums // 10,  % unconditionally before printing the digit.
    dash_num_words(NDiv),
    NMod is Nums mod 10,
    word(NMod),
    write('-').
 
:- num_words(123), nl, num_words(0), nl.

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