Some mud restrict building commands to players whose builder flag is set. The builder flag, BUILDER, is only meaningful for players. On such systems, only builders can @create, @dig, @link, @open, or pick up unlinked exits. Only a wizard can set this flag. When BUILDER is set on a program, it is called "BOUND" and it causes any functions within the program to run in preempt mode, regardless of the multitasking mode that the process had before calling this program. When the execution exits this program, the multitasking mode returns to what it was before the function was called. This lets libraries of atomic functions be written.