some recursive functions don't work as expected

Question

Playing around with recursive preprocessor functions uncovers a strange bug.  I can't quite work out what it is doing, but here's my best attempt at demonstrating:

Take a function f(x) = f(x + 1) + 1 & f(10)=10.  I would expect f(1) = f(2)+1 = f(3)+2 = f(4)+3 = ... = f(10)+9 = 19

But coding it in Plantuml gives the answer 11...  So I stuck in some additional debug info:

@startuml
!function $f($n)
  !$debug=$debug + $n +","
  !if ($n >= 10)
    !return $n
  !else 
    !return $f($n + 1) + 1
  !endif
!end function

!$debug = ""
:f(1) = $f(1);
:debug = $debug;
@enduml

This suggests that the function is getting called only 6 times (instead of 10 times) and each call is n+2 instead of n+1.

My best guess is that 

!return $f($n + 1) + 1

is somehow getting processed as

!return $f($n + 1 + 1)

I can 'fix' it using %intval(), as follows;

@startuml
!function $f($n)
  !$debug=$debug + $n +","
  !if ($n >= 10)
    !return $n
  !else 
    !return %intval($f($n + 1)) + 1
  !endif
!end function

!$debug = ""
:f(1) = $f(1);
:debug = $debug;
@enduml