[Rd] head.matrix can return 1000s of columns -- limit to n or add new argument?

[Michael Chirico]

I think of head() as a standard helper for "glancing" at objects, so I'm
sometimes surprised that head() produces massive output:

M = matrix(nrow = 10L, ncol = 100000L)
print(head(M)) # <- beware, could be a huge print

I assume there are lots of backwards-compatibility issues as well as valid
use cases for this behavior, so I guess defaulting to M[1:6, 1:6] is out of
the question.

Is there any scope for adding a new argument to head.matrix that would
allow this flexibility? IINM it should essentially be as simple to do
head.array as:

do.call(`[`, c(list(x, drop = FALSE), lapply(pmin(dim(x), n), seq_len)))

(with extra decoration to handle -n, etc)

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