# Re: [tlaplus] Which symbols need to be defined? (new to TLA+)

In the example, p and q are symbols which have been used to define the infix operator |. However, they have not been declared or defined before use. Whereas, M, N, x, and y have been defined/declared before using them.  My question is, why do M, N, x, and y require a declaration while p and q don't? Aren't they all symbols?

On Sun, Jun 30, 2019 at 3:22 AM Hillel Wayne <hwayne@xxxxxxxxx> wrote:

p | q == \E d \in 1..q : q = p * d

That's defining the infix operator |, so p and q are the parameters.

On Sun, Jun 30, 2019, 12:31 AM Saswata Paul <paulsaswata1@xxxxxxxxx> wrote:
In page 11 of the TLA+ hyperbook, it is clearly stated that "Every symbol that appears in the module must either be a primitive TLA+ operator or else de fined or declared before its first use.". However, in the example given in the site https://tla.msr-inria.inria.fr/tlaps/content/Documentation/Tutorial/The_example.html, the symbols p and q have not been declared or defined before use. So can someone clarify how we can understand which symbols need to be declared and which do not?

PS: The example code from the site:
-------------------- MODULE Euclid -------------------- EXTENDS Integers p | q == \E d \in 1..q : q = p * d Divisors(q) == {d \in 1..q : d | q} Maximum(S) == CHOOSE x \in S : \A y \in S : x >= y GCD(p,q) == Maximum(Divisors(p) \cap Divisors(q)) Number == Nat \ {0} CONSTANTS M, N VARIABLES x, y Init == (x = M) /\ (y = N) Next == \/ /\ x < y /\ y' = y - x /\ x' = x \/ /\ y < x /\ x' = x-y /\ y' = y Spec == Init /\ [][Next]_<<x,y>> ResultCorrect == (x = y) => x = GCD(M, N) THEOREM Correctness == Spec => []ResultCorrect
=======================================================

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