{ 2 } Push a sequence to the stack containing one element, 2.

Stack: 4 { 2 }

swap Swap two objects (because times needs access to the 4 later.

Stack: { 2 } 4

[ reverse [ 1 + <array> ] map-index concat ] Push a quotation for times to use later. This is a quotation that produces the next row of the Levine's Triangle from the previous one.

Stack: { 2 } 4 [ reverse [ 1 + <array> ] map-index concat ]

times Call a quotation a certain number of times. Inside the quotation now for the first iteration...

Stack: { 2 }

reverse Reverse a sequence.

Stack: { 2 }

[ 1 + <array> ] Push a quotation for map-index to use later.

Stack: { 2 } [ 1 + <array> ]

map-index Take a sequence and a quotation and apply the quotation to each element of the sequence, collecting the results into a new sequence of the same length. However, it also places the index of the element on top of the stack. Inside the quotation now for the first iteration...

Stack: 2 0

1 Push 1.

Stack: 2 0 1

+ Add.

Stack: 2 1

<array> Create an array from a count and an element.

Stack: { 1 1 }

Now map-index has exhausted its elements. Since we are mapping numbers to arrays, the result will be an array of arrays.

Stack: { { 1 1 } }

concat Concatenate a sequence of sequences together into one sequence.

Stack: { 1 1 }

Now we have produced the second row of Levine's Triangle. times will run the quotation three more times...

Stack: { 1 1 2 3 }

Σ Sum a sequence.

Stack: 7

{ 2 } Push a sequence to the stack containing one element, 2.

Stack: 4 { 2 }

swap Swap two objects (because times needs access to the 4 later).

Stack: { 2 } 4

[ reverse [ 1 + <array> ] map-index concat ] Push a quotation for times to use later. This is a quotation that produces the next row of the Levine's Triangle from the previous one.

Stack: { 2 } 4 [ reverse [ 1 + <array> ] map-index concat ]

times Call a quotation a certain number of times. Inside the quotation now for the first iteration...

Stack: { 2 }

reverse Reverse a sequence.

Stack: { 2 }

[ 1 + <array> ] Push a quotation for map-index to use later.

Stack: { 2 } [ 1 + <array> ]

map-index Take a sequence and a quotation and apply the quotation to each element of the sequence, collecting the results into a new sequence of the same length. However, it also places the index of the element on top of the stack. Inside the quotation now for the first iteration...

Stack: 2 0

1 Push 1.

Stack: 2 0 1

+ Add.

Stack: 2 1

<array> Create an array from a count and an element.

Stack: { 1 1 }

Now map-index has exhausted its elements. Since we are mapping numbers to arrays, the result will be an array of arrays.

Stack: { { 1 1 } }

concat Concatenate a sequence of sequences together into one sequence.

Stack: { 1 1 }

Now we have produced the second row of Levine's Triangle. times will run the quotation three more times...

Stack: { 1 1 2 3 }

Σ Sum a sequence.

Stack: 7