RPG: Control flow statements "IF", "FOR", "DO", ... should not be nested too deeply

Nested CASxx, DO, DOU, DOW, DOUxx, DOWxx, FOR, IF, IFxx, MONITOR and SELECT statements is a key ingredient for making what's known as "Spaghetti code".
Such code is hard to read, refactor and therefore maintain.

Noncompliant Code Example

With the default threshold of 3:

IF        A <> B
IF        B <> C
DOW       B <> A
IF        A <> D
...
ENDIF
ENDDO
ENDIF
ENDIF

if a <> b;
  if b <> c;
    dow b <> a;
      if a <> d;
        ...
      endif;
    enddo;
  endif;
endif;