Merge assumptions about external functions

Author: mikucionisaau

content/language-reference/system-description/declarations/external-functions.md

@@ -12,7 +12,15 @@ The external functions are supported since Uppaal Stratego version 4.1.20-7, or
   The feature is supported on Linux and experimental on Windows and macOS.
 {{% /notice %}}
 
-External Functions can be declared alongside other declarations. External functions are local to the current scope, defined by the grammar:
+{{% notice warning %}}
+  The external function calls must be ***deterministic***: for any argument values, the function should produce the same result at any time when called with the same values.<br>
+  For example:
+  - *Side-effect-free* (or *free*) functions, which do not depend on [`static`](https://en.wikipedia.org/wiki/Static_variable) variables, are *deterministic*.
+  - [Memoization](https://en.wikipedia.org/wiki/Memoization) pattern is not *side-effect-free* but results in a *deterministic* behavior, hence is also allowed.
+{{% /notice %}}
+
+External functions can be declared alongside other declarations. 
+External functions are local to the current scope, defined by the grammar:
 
 ``` EBNF
 ExternDecl   = 'import'  Path '{' [FwdDeclList] '}'