Following nice suggestions given in response to my earlier query, I used to be inspired to think about a special sort of heap construction, the Leftist Tree. It lends itself higher to a useful implementation. It is already shorter, however is there anything I ought to think about with the intention to make this extra idiomatic APL? Many thanks.

⍝ APL implementation of a leftist tree.
⍝ 
⍝ https://en.wikipedia.org/wiki/Leftist_tree
⍝ http://typeocaml.com/2015/03/12/heap-leftist-tree/

⎕io←0
Insert←{ ⍝ Insert merchandise into leftist tree, returning the ensuing tree
    (tree merchandise)←⍵
    1 merchandise ⍬ ⍬ Merge tree 
}

Pop←{ ⍝ Pop off smallest factor from a leftist tree
    0=≢⍵:⍬
    (v l r)←1↓⍵                 ⍝ worth left proper
    (l Merge r) v               ⍝ Return the ensuing tree and the worth
}

Merge←{ ⍝ Merge two leftist bushes, t1 and t2
    t1←⍺ ⋄ t2←⍵
    0=≢t1:t2 ⋄ 0=≢t2:t1                          ⍝ If both is a leaf, return the opposite
    (key1 left proper)←1↓t1 ⋄ key2←1⌷t2
    key1>key2:t2∇t1                              ⍝ Flip to make sure smallest is root of merged
    merged←proper∇t2                              ⍝ Merge rightwards
    (⊃left)≥⊃merged:(1+⊃merged) key1 left merged ⍝ Proper is shorter
    (1+⊃left) key1 merged left                   ⍝ Left is shorter; make it the brand new proper
}

⍝ Instance heap merge from http://typeocaml.com/2015/03/12/heap-leftist-tree/
h←Insert ⍬ 2
h←Insert h 10
h←Insert h 9

s←Insert ⍬ 3
s←Insert s 6

h Merge s
┌→─────────────────────────────────────────────────────────────┐
│     ┌→────────────────────────────────────┐ ┌→─────────────┐ │
│ 2 2 │     ┌→────────────┐ ┌→────────────┐ │ │      ┌⊖┐ ┌⊖┐ │ │
│     │ 2 3 │     ┌⊖┐ ┌⊖┐ │ │     ┌⊖┐ ┌⊖┐ │ │ │ 1 10 │0│ │0│ │ │
│     │     │ 1 6 │0│ │0│ │ │ 1 9 │0│ │0│ │ │ │      └~┘ └~┘ │ │
│     │     │     └~┘ └~┘ │ │     └~┘ └~┘ │ │ └∊─────────────┘ │
│     │     └∊────────────┘ └∊────────────┘ │                  │
│     └∊────────────────────────────────────┘                  │
└∊─────────────────────────────────────────────────────────────┘