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*From*: Saswata Paul <paulsaswata1@xxxxxxxxx>*Date*: Sat, 29 Jun 2019 22:31:14 -0700 (PDT)

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 defined 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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**Follow-Ups**:**[tlaplus] Re: Which symbols need to be defined? (new to TLA+)***From:*Leslie Lamport

**Re: [tlaplus] Which symbols need to be defined? (new to TLA+)***From:*Hillel Wayne

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