Revert addition of 'slip' to S04

'slip' may (or may not) be a nice+useful feature, but we're not ready to
add it to core just now. In GLR we have more pressing concerns.

This reverts commit 24850d6b665913be797067a5c80e8d3fdfc03c1b.
This reverts commit e4c5b10b3bed23d270f50208b9e4c621c18dacba.