┌┬┐├┼┤└┴┘│─

┌   &#x250c;   &#9484;   upper left corner
┬   &#x252c;   &#9516;   top
┐   &#x2510;   &#9488;   upper right corner
├   &#x251c;   &#9500;   left
┼   &#x252c;   &#9516;   middle
┤   &#x2524;   &#9508;   right
└   &#x2514;   &#9492;   lower left corner
┴   &#x2534;   &#9524;   bottom
┘   &#x2518;   &#9496;   lower right corner
│   &#x2502;   &#9474;   north-south
─   &#x2500;   &#9472;   east-west

For example:

<"0 i. 3 4
┌─┬─┬──┬──┐
│0│1│2 │3 │
├─┼─┼──┼──┤
│4│5│6 │7 │
├─┼─┼──┼──┤
│8│9│10│11│
└─┴─┴──┴──┘

The box drawing characters are also useful for the tree representation (5!:4). The following example uses verbs from the Matrix Inverse page.

minors =: 1&(|:\.)"2^:2
det    =: -/ .*
adjoint=: [: |: */~@(\$&1 _1)@# * det@minors

treerep=: 1 : '5!:4 <''u.'''

inverse f. treerep
┌─ [:
├─ |:
│                  ┌─ ~ ─── / ──── *
│                  │
┌─────┤           ┌─ @ ──┤     ┌─ \$
│     │     ┌─ @ ─┤      └─ & ─┴─ 1 _1
│     │     │     └─ #
│     │     ├─ *
│     └─────┤            ┌─ / ─── -
│           │     ┌─ . ──┴─ *
──┤           │     │                   ┌─ 1
│           └─ @ ─┤            ┌─ & ──┴─ \. ─── |:
│                 │      ┌─ " ─┴─ 2
│                 └─ ^: ─┴─ 2
├─ %
│     ┌─ / ─── -
└─ . ─┴─ *