# Running old scripts unchanged

For compatibility with old codes, MODELLER will still run most TOP scripts. By default, the program expects to read new-style Python scripts, but if the script file extension ends in `'.top'` (as in previous versions), it will be assumed to be a TOP file and will be run as such. Note, however, that this behavior is deprecated and will probably be removed entirely in a later release. Also bear in mind that some newer commands and features will not be available in TOP; thus, it is recommended that you convert your TOP scripts to Python.

If you do wish to run old scripts unchanged, please note that the GO_TO function is no longer present in the TOP language. If you have code which makes use of this function, you should use the EXIT and CYCLE flow control statements instead, which either terminate a DO loop or skip to its next iteration. Single-line IF statements are no longer supported either; you should use the ELSE and END_IF statements to build multi-line IF clauses instead. See the scripts in the `bin` directory for examples.