What is the feature or improvement you would like to see?
Expose igraph_motifs_randesu_callback() in R, so that I can draw a random sample of fully connected triads (A-B-C-A) and partially connected triads (A-B-C) from a large graph object (15m nodes, 60m edges)

Use cases for the feature
For the statistical analysis of the properties of a large social network, I want to report how certain node characteristics vary across triads depending on whether they are closed or not. The network is way too large to enumerate all those networks to use a manual search for all triads. Also, a sample would already be sufficient.

References
List any relevant references (papers or books describing relevant algorithms).