Reserved keywords

The placeholder atom

The _ atom can be used in place of an argument to a selector to indicate that you don't care about it. For example, you can use it in the (arg) selector to make it match a call to x with an argument at any position that is a number:

(call x (arg _ (num)))

Or, when you don't care about the callee but only the arguments with which it was called:

(call _ (arg _ (num)))

Technically, _ tells SYNG to entirely skip the selection of whatever the argument is meant to select. It cannot, however, be used in place of required arguments.