Question: select and the use of $

Dear people in mapleprimes,


I have a question which is probably very elementary.

> integers:=[10..15]:
> select(isprime, integers);
Then, "Error, selecting function must return true or false" appears.

On the other hand, 

> integers:=[$10..15]:
> select(isprime, integers);

Then , [11, 13] appears.


What role does the character "$" play here? 

 Thank you in advance.



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