Some libraries export variables that are used and modified internally, but which would cause problems if modified externally. It should be possible to export these variables as read-only so that they look like constants in any file which includes them. Two suggestions for the syntax could be, for example:
export sequence as constant my_variable -- or:
export sequence as readonly my_variable
Implementers are likely to have better ideas for the syntax.
I would not expect this before version 5.
Arthur