Let e be an expression (i.e., e is a standing for some expression). Let e rightarrow e' be a that asserts a between two expressions, saying that e' Meanwhile 1 + 2 + 3 rightarrow 3 + 3 is an example of a relation derivation judgment inference rule judgment form meta - variable programs symbols expressions objects Consider the form of a(n) J1, J2, ... J_n/J The judgments on top of the horizontal line are called The below the horizontal line is called the