To be useful, persistent variables must be mutated. Persistents can only be mutated in the postlude.
The following statements mutate persistent variables:
clear <pvar>. This deletes the persistent variable<pvar>.<pvar> := <expr>. This assigns the value of <expr> to <pvar>.
If the persistent variable is bound to a complex data structure (i.e. maps and arrays),
<pvar> can contain a hash path referencing part of the structure. Example
fired {
ent:a := 1;
app:b{"foo"} := 1 + my_var.defaultsTo(0);
app:b{["flip", "flop"]} := "this is a string";
ent:c := null
clear ent:a
}