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A short exact sequence is a sequence of maps:
       f     g     h     k
    0 --> A --> B --> C --> 0
s.t.
 Im f = Ker g
 Im g = Ker h
 Im h = Ker k

Example:

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/*
* This is a literate quine. That means that
* 1. the comments will tell you a little about how it works and
* 2. if you compile and run it its output will be identical to its source
*    code even though it doesn't look at its original source. It literally
*    contains within itself a complete recipe for how to display itself.
*
* Quines are ten a penny. This one is unusual because
* 1. its main loop consists solely of a loop to print characters
*    generated by a function called programChar() and

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# Python 3
import collections
Write = collections.namedtuple("Write", ["written"])

def hello_world():
    yield Write("Hello, world!")
    yield Write("Hello, world!")
    return 1

def identity(gen):

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import csv
import math
import matplotlib.pyplot as plt
import locale

def frac(x):
    return x - math.floor(x)

def first_digit(x):
    return int(math.floor(math.pow(10, frac(math.log10(x))) + 0.0001))

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Here's a block of code in some imaginary assembly language:

   jmp A
   block1
   jmp A
   block2
   jmp A
   block3
.A ...

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# From https://www.python-mip.com/
from mip import *

m = Model(sense = MAXIMIZE)

num_slots = 10
num_workers = 5
num_tasks = 3

def slot_to_time(slot):

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// Some platforms import from experimental/coroutine
// rather than coroutine.
#define REQUIRES_EXPERIMENTAL 1

// Controls whether the coroutine starts in a suspended state
// or runs to the first suspension point on construction.
// The two implementations should match.
#ifndef INITIAL_RUN
#define INITIAL_RUN 1
#endif

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import math

import numpy as np
import random
import matplotlib.pyplot as plt

medium_size = 1024
x = np.linspace(-1., 1., medium_size)
y = np.linspace(-1., 1., medium_size)
x, y = np.meshgrid(x, y, indexing='ij')

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#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <time.h>

int bernoulli(float p, float r)
{
  return r > 1 - p;
}

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(** User Mathematica initialization file **)

(** See https://reference.wolfram.com/language/tutorial/ConfigurationFiles.html for info on instaling this file **)

(** Display graphics inline in iTerm2. I don't know an easy way to test if we're running in iTerm2 without looking at the process table. **)

imgcat[image_Graphics]:=(
  WriteString[$Output, "\033]1337;File=inline=1:"<>ExportString[ExportString[image,"PNG"],"Base64"]<>"\007"];
  Null
)