|kernel:set-symbol-value-conditional symbol old new||[Function]|
Atomically compare symbol's value to old and if eq then store new in symbol's value cell. The original symbol value is returned. If the symbol has a binding in the current thread then the symbol dynamic value cell is accessed, otherwise the symbol global value cell is accessed. See also: kernel:set-symbol-value-conditional and kernel:set-symbol-global-value-conditional.