My idea was to divide your layout into small parts, each one with one metapost definition of user symbol and use the command in thconfig file:

input - works like input command in data files - includes other files.

copy <source layout id> . set properties here that are not modied based on the given source layout. - in your layout.

file: layout_foo:

layout u_foo
  code metapost
    definition of u:foo
  endcode
endlayout

thconfig file:

input layout_foo

layout my_layout
   #copy u_foo #this is the line which could be commented 
endlayout

export foo_off

it was not tested prior post!

m.

Sep 13, 2013 v 8:27 AM, Xavier Pennec <Xavier.Pennec@inria.fr>:

Well, I gathered my layout definitions in a single file to avoid repeating and duplicating everything in my 50 thconfig files. This allows me to modify only one layout file to modify consistently all the layouts of all caves. I think heritage + redefinition is a more structured