\x .z.p*

By default, callbacks like .z.po are not defined in the session. After they have been assigned, you can restore the default using \x to delete the definition that was made.

q).z.pi                       / default has no user defined function
'.z.pi
q).z.pi:{">",.Q.s value x}    / assign function
q)2+3
>5
q)\x .z.pi                    / restore default

Works only for .z.p* variables defined in k before q.k is loaded

For example, as .z.ph is defined in q.k, there is no default for it to be reset to.