[master] Tests for "when"

[master] "when" without tests

Merge remote-tracking branch 'origin/additional-stacks'

[additional-stacks] Stack initializing

[additional-stacks] Rename set-stack to stack

[additional-stacks] Remove unnecessary variables

[additional-stacks] Remove unused

[additional-stacks] Pop stack

Different stacks

[master] Simple word definition

[master] Move eval code to Core

Merge pull request #9 from couchemar/move-vocabs-to-context

Fix existing stack tests

Update define word

Update defspecial