Not sure that this is exactly what you are looking for, but you can use @pause / @unpause to pause and resume PlantUML text diagram.
For example:
// @startuml
// start
// :initiaize;
// @pause
var a=1;
var b=2;
// var c=3; not in plantuml because "//" after with space
// @unpause
// :run the logic;
// @pause
a=b;
// @unpause
// end
// @enduml
Does is help ?