moveBack abstract method
Moves the range to be the last count characters before the current
range.
The new range ends at the start of the current range and includes
the previous count characters, or as many as available if there
are fewer than count characters preceding the current range.
The count must not be negative.
If it is zero, the call has the same effect as collapseToStart.
Returns true if there were count preceding characters
and false if not.
Implementation
bool moveBack([int count = 1]);