function A takes nothing returns nothing
endfunction
function B takes unit u returns nothing
endfunction
function C takes nothing returns boolean
endfunction
function main takes nothing returns nothing
local group MyGroup = CreateGroup()
call ForGroup(MyGroup, function A) // valid
call ForGroup(MyGroup, function B) // not valid, function B has arguments
call ForGroup(MyGroup, function C) // not valid, function C returns a value
endfunction