Changed concealing of elem and notElem to their infix form

``` `elem` ``` and ``` `notElem` ``` seem intuitively right to be replaced

Previous form was especially misleading when used as section `(elem a)` would become `∈ a` which is not very correct looking.

![infix_elem](https://cloud.githubusercontent.com/assets/5804169/3004412/d22ac02c-dd99-11e3-8c09-c695c4a28ad3.png)
.