code: public Assignment model(final boolean defaultValue, final Collection<Variable> variables) {
      return this.factory.model(this, variables, defaultValue);
    }
docs: Returns an arbitrary model of this BDD which contains at least the given variables or {@code null} if there is
  none.  If a variable is a don't care variable, it will be assigned with the given default value.
  @param defaultValue the default value for don't care variables
  @param variables    the set of variable which has to be contained in the model
  @return an arbitrary model of this BDD