ConditionalSet should support extended definition

### Currently
Currently, it has `Var` and `Predicate`, for example, `{ x : x > 0 }`. But what if we need every element to be `f(x)`?
In math we would write
`{ y : there exists x such that x > 0 and y = f(x) }`
Although it's correct, it's inconvenient for computers (and quantifiers are not yet supported).

### Proposal
To add the `Transformation` for every element
`{ f(x) : x : x > 0}` - the syntax is yet to be discussed, but the general idea should be clear.
`{ x : x : p(x)}` is equivalent to current `{ x : p(x) }` (the old syntax will be kept).

### Usefulness
This will help to close #322 and #318.