Before pcb-rnd switched to lihata for native file format, we used to use
geda/pcb's format as native. Since pcb-rnd got a lot of new features and
since geda/pcb refused to cooperate, there are unfortunately some 
non-intrusive local file format extensions to the geda/pcb format.

These extensions are kept for backward compatibility: just in case a file
needs to be loaded that was produced with pre-lihata pcb-rnd. However,
new extensions are not added to the geda/pcb format.

This file attempts to (vaguely) list the extensions. Unless otherwise
mentioned, geda/pcb will (often silently) ignore the extension and a
load-save with geda/pcb will remove the extended properties from the file
(while pcb-rnd still does proper round trips on them).

1. .fp file: element tagging

 Any comment in an element file that starts with double # is taken
 as a tag. The syntax is: ##key:value. A tag is basically the same
 as an attribute, except tags are not becoming subcircuit attributes
 after loading a footprint. Tags are used to ease footprint searches.

2. .pcb and .fp file: pin shape

 Early in pcb-rnd development, years before padstacks, a minor feature
 was added to allow 16 different asymmetric pin shapes. The pin shape
 is saved as a flag.

3. .pcb file: intconn

 Because elements couldn't have attributes, we abused the flag field for
 storing intconn() info. 

4. .pcb file: netlist patch

 For back annotation pcb-rnd keeps track on deliberate netlist changes.
 Since this feature happened before lihata got stable, it is also
 saved in the old pcb format in a NetListPatch subtree. Unfortunately
 pcb does NOT handle this and throws a syntax error. geda/pcb users are
 required to remove this subtree manually.

5. .pcb file: new setting: enablemincut

 Another of the first few new features in pcb-rnd was mincut for shorts.
 This also happened years before lihata boards, so a new global
 setting was registered, which may even be saved in a board file.