test.rs

  fn parse_term<'i>(input: &'i mut ParserStream<'i>) -> Result<Option<LinearExpression>, String> {
    let coef = Parser::parse_scalar(input).unwrap();
    input.skip_whitespace();
    let var = Parser::parse_identifier(input).unwrap();
    match (coef, var) {
      (Some(scalar), Some(var)) => Ok(Some(LinearExpression::term(var, scalar))),
      (Some(scalar), None) => Ok(Some(LinearExpression::from(scalar))),
      (None, Some(var)) => Ok(Some(LinearExpression::from(var))),
      (None, None) => Ok(None)
    }
  }