Check if it is a valid string before appending it

Check if the string to be added will create a valid expression, if it
does it will create the new string. If it will not it returns the
original string.
Fix return of original string

When trying to add, if it will not create a valid expression, return
the original string.
Merge pull request #2 from aparedes/master

Minor fixes to stringFrom:withThis:here: