do

(do ((var init step?)...) (end-test result...) body...)

Iterates with one or more loop variables. Each var is bound to its init on entry and, after every pass through the body, updated in parallel to its step form (every step is evaluated against the old bindings before any variable is reassigned). Before each iteration end-test is checked; when it becomes truthy the loop stops and the result forms are evaluated, the last value being returned (nil if there are none). The body and steps are wrapped in the internal block boundary, so return exits the loop early.