1. override
Expression operator +(EquationMember m)

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

For example: (left + right) / cm(2.0) can be used as an Expression equivalent of the midPointX 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(m), constant);

  if (m is Expression) {
    return new Expression(
      new List<Term>.from(terms)..addAll(m.terms),
      constant + m.constant
    );
  }
  assert(false);
  return null;
}