1. override
Expression operator -(EquationMember m)

Creates a Expression by subtracting the argument from this member. Both members may need to be hoisted to expressions themselves before this can occur.

For example: right - left can be used as an Expression equivalent of the width property.

Source 

@override
Expression operator -(EquationMember m) {
  if (m is ConstantMember)
    return new Expression(new List<Term>.from(terms), constant - m.value);

  if (m is Param) {
    return new Expression(
      new List<Term>.from(terms)..add(new Term(m.variable, -1.0)),
      constant
    );
  }

  if (m is Term) {
    return new Expression(new List<Term>.from(terms)
      ..add(new Term(m.variable, -m.coefficient)), constant);
  }

  if (m is Expression) {
    List<Term> copiedTerms = new List<Term>.from(terms);
    for (Term t in m.terms)
      copiedTerms.add(new Term(t.variable, -t.coefficient));
    return new Expression(copiedTerms, constant - m.constant);
  }
  assert(false);
  return null;
}