.html">CpModelProto</a> variables field.
Depending on the context, a reference to a variable whose domain is in [0, 1]
can also be seen as a Boolean that will be true if the variable value is 1
and false if it is 0. When used in this context, the field name will always
contain the word "literal".
Negative reference (advanced usage): to simplify the creation of a model and
for efficiency reasons, all the "literal" or "variable" fields can also
contain a negative index. A negative index i will refer to the ne