In some tutorials for certain "Maple-words" a name "command" is used, while others use "function".

What is the difference between "function" and "command" in Maple?

Is there a possibility in Maple to define a set of elements using a rule or semanticdescription. The problem is that  I need to define a set of numbers of the concrete form (e.g. of the form a+b\sqrt[3](2)). Or more complecated task: the set of pairs (a,b) where a,b::integer. And then I need to define an abstract operation on that set. I explain: my global task is to create a procedure to define whether a set with an operation is a group (where a set is defined using a rule).