>>217に続きがあった

Then you are ready to do research with these tools.
For instance one of our research professors tells his students the prerecquisite for
working in algebraic number theory is to become comfortable with algebra at the level of Lang.
But our course in PhD prelim preparation for algebra will probably use Dummitt and Foote,
just because it is a more feasible book for the students to read at that stage.
Attempts to use Lang in trhe past have been disastrous.

Nonetheless, even students who found Lang a frustrating text,
still use it as a necessary reference, and even find it has too little.

Just compare the treatment of groups in Lang and Dummitt and Foote.
Lang covers the whole subject in more depth in 60 pages (2nd edition)
while D/F use up over 220 pages on groups, and still do not introduce
the categorical point of view, and in particular do not prove the existence of
"direct sums" i.e. coproducts (which they do not even define), of groups.

So if you only have Lang, you will almost surely not see enough detail to understand
the material, and if you only have D/F you will not see it from quite the right perspective,
and will still not know some basic results.