Version 1.5.1 added threaded functions. These functions are very similar to lambda functions except that whenever they are called they will run in a different thread. ZPE 1.7.9 changed threaded functions so that they are no longer as complex as they were previously by adding the run_in_thread function.
From version 1.7.1 to 1.7.8, threaded functions are written as:
function threaded foo($x) print ($x + 5) end function foo(3)
Threaded functions will operate in a different thread. This is useful particularly for server operations added in version 1.5.0.
Prior to version 1.7.1 threaded functions were written as:
threaded function foo($x) print ($x + 5) end function foo(3)