This command is used to apply a transformation (described by body) to the dictionaryValue to produce a new dictionary. It is analogous to dict for as foreach is to lmap.
The new value created in the result dictionary will be the result of evaluating body that time round the loop, and the key used will be the current value of the variable keyVar (i.e., change the variable to change the key). It's an error to have the variable unset at that point; skip a round by using continue (or stop early with break).