Input tab characters are rejected except at the "outermost" level of lexical grouping.  (eqn is not fooled by superfluous braces.)

Since leaders are pretty much "noisy" tabs, they should be handled similarly.

$ hd EXPERIMENTS/eqn-using-tab.roff
00000000  2e 45 51 0a 61 09 62 20  73 71 72 74 20 7b 20 63  |.EQ.a.b sqrt { c|
00000010  09 64 20 7d 0a 2e 45 4e  0a                       |.d }..EN.|
00000019
$ ./build/eqn -Tascii EXPERIMENTS/eqn-using-tab.roff >/dev/null
./build/eqn:EXPERIMENTS/eqn-using-tab.roff:1: error: tabs allowed only at outermost level
$ hd EXPERIMENTS/eqn-using-leader.roff
00000000  2e 45 51 0a 61 01 62 20  73 71 72 74 20 7b 20 63  |.EQ.a.b sqrt { c|
00000010  01 64 20 7d 0a 2e 45 4e  0a                       |.d }..EN.|
00000019
$ ./build/eqn -Tascii EXPERIMENTS/eqn-using-leader.roff >/dev/null
[bupkis; no diagnostic]

And indeed things go pretty wrong if you sneak a leader past eqn.

$ ./build/test-groff -e -Tascii EXPERIMENTS/eqn-using-leader.roff | cat -s
troff:EXPERIMENTS/eqn-using-leader.roff:20: warning: special character 'sqrt' not defined
troff:EXPERIMENTS/eqn-using-leader.roff:49: warning: special character 'rn' not defined
troff:EXPERIMENTS/eqn-using-leader.roff:2: error: invalid token in argument to escaped 'Z'
troff: error: horizontal line drawing character must have positive width
a.......bcd

The code change would happen around here.