Content Binding by Pattern Match

this is using AsTMa! patterns
always 'all-quantified', i.e. patterns are applied as many times as possible
this means we implicitely generate a sequence of these matches
```
forall [ $d is-a document ]
```
constraint based loop

every match makes all variables mentioned in the pattern be bound to a value
structural variables are bound to 'items', i.e. (c)maplets
string variables are bound to ... strings
variables for simple things $a are bound only to one value at a time
```
forall $a [ * is-a document ] ...
```
variables for list things @s are bound to all values in the match
```
forall @a [ * is-a document ]
```