Extended elevation, change of height only

[Andrew Atkinson]

Hi all

When doing extended elevation Therion has a command that only uses the 
vertical component of of the leg. This is very useful for legs that 
cross a passage and some avens.

pp101 of the Therion book
extend vertical 5 5’

As far as I can tell survex does not have this functionality

https://survex.com/docs/manual/extend.htm

Am I missing something

Reason I am asking is that I am implementing the vertical feature in ST, 
assuming it is accepted. That would mean the export to survex would not 
be able to do the same. Currently the extend part of survex is broken so 
this is a future issue.

Thanks

Andrew