(tested in latest commit)

When you run slice(str,index), the result will be missing the last character. Using length(str) as end position makes it return the empty string.

slice('helloworld', 2) -> 'lloworl'
str = 'helloworld'; slice(str, 2) -> 'lloworld'
str = 'helloworld'; slice(str, 2, length(str)) -> ''
str = 'helloworld'; slice(str, 2, length(str) - 1) -> 'lloworl'

It was introduced in 7ad6de6 (worked correctly after locally reverting that commit).

oups