Allow stacking filter and keep operations

[sehrgut]

A common house rule for D&D stats is 4d6r1k3 (4d6 reroll 1s keep 3 highest).

I would expect this to be notated as 4d6>1k3. However, these options do not stack, so I'm left rolling 4{d6>1} to see separate rolls, and adding them myself.

Allowing the grammar to fire off multiple filter and keep operations (as well as arithmetic operations) would allow this, as well as rolls where both some number of highest and lowest dice need to be rerolled or kept, e.g.:

8d6-H-H-L-L (roll 8d6 dropping the 2 highest and two lowest)
3d6>1<6 (roll 3d6 rerolling 1s and 6s)

[matteocorti]

This could be implemented, I'll leave it here as "enhancement" until I find the time or someone volunteer to implement the change.