assert←{⍺←'assertion failure' ⋄ 0∊⍵:⍺ ⎕signal 8 ⋄ shy←0}

tassert ← {
  assert (1≤≢⍵)∧1=⍴⍴⍵ :  ⍝ non-empty vector
  assert (⊃=⊢)≢¨⍵ :      ⍝ equal tally in each item
  assert 2=≡⍵ :          ⍝ nested array with simple items
  1
}

   tassert T
1
   tassert X
assertion failure
tassert[1] assert(1≤≢⍵)∧1=⍴⍴⍵:  ⍝ non-empty vector
           ∧