Wafer {
    Main = Expr
    Expr = PrimaryExpr (op PrimaryExpr)*

    PrimaryExpr = "(" Expr ")"  -- paren
                | number

    op = "+" | "-" | "*" | "/"
    number = digit+

    // Examples:
    //+ "42", "1", "66 + 99", "1 + 2 - 3", "1 + (2 * 3)", "(((1) / 2))"
    //- "abc"
  }