[igraph] What is the max value of a node id?

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Hello everyone,

I have following codes by c++:

igraph_empty(&g,0,true);

igraph_add_vertices(&g,131071,0);//17

igraph_add_vertices(&g,262143,0);//18

igraph_add_vertices(&g,524287,0);//19

igraph_add_vertices(&g,1048575,0);//20

igraph_add_vertices(&g,2097151,0);//21

igraph_add_vertices(&g,4194303,0);//22

igraph_add_vertices(&g,8388607,0);//23

igraph_add_vertices(&g,16777215,0);//24

igraph_add_vertices(&g,16777216,0);//24+1 does not work

as you can see, I'm testing the max value available of node id.

but why it does not work when node id is longer than 24 bits?