resolved override conflict
merge with develop
Merge branch '2-3-stable'
Merge branch '2-4-stable'
Merge branch '2-4-stable'
Merge branch '2-4-stable'
Merge branch '2-4-stable'
Merge branch '2-4-stable'
Merge branch '2-4-stable'
Merge branch '2-4-stable'
improving calculator
bug fix