How to define a set of paths, or set of sequences of links, in AMPL?

I'm trying to define a structure to capture something like below:

set NODES := A B C;
set LINKS := (A,B) (B,C);
set PATHS := ((A,B)) 
             ((A,B), (B,C))
             ((B,C));

Nodes are a set. Links are a set of node pairs.

I am having trouble defining Paths as a set of sequences of links. I have not seen any solutions in the AMPL graph examples that make explicit use of paths, and I am wondering if there is a simple way to construct them?

Here are the definitions in my .mod file:

set NODES; 
set LINKS within (NODES cross NODES);
set PATHS # ??? ;

Please help.

Given that your paths do not have repeated nodes, the most natural way I can think of to define paths would be as a collection of ordered sets of nodes.

reset;
model;
set NODES;
set LINKS within {NODES,NODES};

param n_paths;
set PATHS{1..n_paths} within NODES ordered;

# Optional: identify all of the links implied by these paths, so we can
# check that they are in fact within LINKS. 

param longest_path_length := max{i in 1..n_paths} card(PATHS[i]);
set LINKS_IMPLIED_BY_PATHS within LINKS := setof{
      i in 1..n_paths, 
      j in 1..(longest_path_length-1): j < card(PATHS[i])
    } (member(j,PATHS[i]),member(j+1,PATHS[i]))
    ;

data;
set NODES := A B C;
set LINKS := (A,B) (B,C);

param n_paths := 3;
set PATHS[1] := A B;
set PATHS[2] := A B C;
set PATHS[3] := B C;

display LINKS_IMPLIED_BY_PATHS;
# if we include a path like C A, we will get an error here because ("C","A") 
# is not within LINKS.
# It should be possible to do this more tidily with a check statement but
# for the moment the syntax escapes me.
# Note that this error will ONLY appear at the point where we try to
# do something with LINKS_IMPLIED_BY_PATHS; it's not calculated or checked
# until then. 

This isn't quite what you asked for, since it defines paths as a sequence of nodes rather than links, but it's the closest I could get.