Advent of code 2020. Day 1, part b. PostScript version.

aoc1b.ps

[ (input.txt) run ]             % A
dup length 1 sub                % L-1 A
0 exch                          % L-1 0 A 
1 exch                          % L-1 1 0 A
{                               % i A
    exch                        % A i 
    1 index 1 add               % i+1 A i        
    1                           % 1 i+1 A i 
    2 index                     % A 1 i+1 A i 
    length 1 sub                % L-1 1 i+1 A i  
    {                           % j A i (invariant i < j)
        exch                    % A j i
        1 index 1 add           % j+1 A j i 
        1                       % 1 j+1 A j i
        2 index                 % A 1 j+1 A j i
        length 1 sub            % L-1 1 j+1 A j i
        {                       % k A j i (invariant j < k)
            1 index             % A k A j i
            exch                % k A A j i 
            get                 % [k] A j i
            exch                % A [k] j i
            2 index             % j A [k] j i
            1 index             % A j A [k] j i
            exch                % j A A [k] j i
            get                 % [j] A [k] j i
            exch                % A [j] [k] j i
            4 index             % i A [j] [k] j i
            1 index             % A i A [j] [k] j i
            exch                % i A A [j] [k] j i
            get                 % [i] A [j] [k] j i 
            exch                % A [i] [j] [k] j i
            4 1 roll            % [i] [j] [k] A j i
            3 copy              % [i] [j] [k] [i] [j] [k] A j i
            add add             % [i]+[j]+[k] [i] [j] [k] A j i
            2020                % 2020 [i]+[j]+[k] [i] [j] [k] A j i
            eq                  % =? [i] [j] [k] A j i
            {                   % [i] [j] [k] A j i (found it!)
                mul mul         % [i]*[j]*[k] A j i
                =               % A j i
            }   
            {                   % [i] [j] [k] A j i (no match)
                pop pop pop     % A j i
            } ifelse    
        } for                   % A j i
        exch pop                % A i
    } for                       % A i 
    exch pop    
} for                           % A 
pop                             %