```
g1 <- make_empty_graph(n=10)
g2 <- make_full_graph(n=10)
g3 <- sample_gnp(n=10, 0.4)
# loop edges
g <- graph( c(1,2, 2,2, 2,3) )
edge_density(g, loops=FALSE) # this is wrong!!!
edge_density(g, loops=TRUE) # this is right!!!
edge_density(simplify(g), loops=FALSE) # this is also right, but different
```

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