Link to the book for clarification of generator and guard control structures

Author: Ali Hong

_tour/for-comprehensions.md

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-Scala offers a lightweight notation for expressing _sequence comprehensions_. Comprehensions have the form `for (enumerators) yield e`, where `enumerators` refers to a list of enumerators. An _enumerator_ is either a generator which introduces new variables, or it is a guard. A comprehension evaluates the body `e` for each binding generated by the enumerators and returns a sequence of these values.
+Scala offers a lightweight notation for expressing _sequence comprehensions_. Comprehensions have the form `for (enumerators) yield e`, where `enumerators` refers to a list of enumerators. An _enumerator_ is either a generator, or it is a guard (see: [Control Structures](/scala3/book/control-structures.html)). A comprehension evaluates the body `e` for each binding generated by the enumerators and returns a sequence of these values.
 
 Here's an example: