CWEB is an actively used programming language created in 1987.

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Example code from Linguist:

\datethis
@*Intro. This program generates clauses for the transition relation
from time $t$ to time $t+1$ in Conway's Game of Life, assuming that
all of the potentially live cells at time $t$ belong to a pattern
that's specified in |stdin|. The pattern is defined by one or more
lines representing rows of cells, where each line has `\..' in a
cell that's guaranteed to be dead at time~$t$, otherwise it has `\.*'.
The time is specified separately as a command-line parameter.

The Boolean variable for cell $(x,y)$ at time $t$ is named by its
so-called ``xty code,'' namely by the decimal value of~$x$, followed
by a code letter for~$t$, followed by the decimal value of~$y$. For
example, if $x=10$ and $y=11$ and $t=0$, the variable that indicates
liveness of the cell is \.{10a11}; and the corresponding variable
for $t=1$ is \.{10b11}.

Up to 19 auxiliary variables are used together with each xty code,
in order to construct clauses that define the successor state.
The names of these variables are obtained by appending one of
the following two-character combinations to the xty code:
\.{A2}, \.{A3}, \.{A4},
\.{B1}, \.{B2}, \.{B3}, \.{B4},
\.{C1}, \.{C2}, \.{C3}, \.{C4},
\.{D1}, \.{D2},
\.{E1}, \.{E2},
\.{F1}, \.{F2},
\.{G1}, \.{G2}.
These variables are derived from the Bailleux--Boufkhad method
of encoding cardinality constraints:
The auxiliary variable \.{A$k$} stands for the condition
``at least $k$ of the eight neighbors are alive.'' Similarly,
\.{B$k$} stands for ``at least $k$ of the first four neighbors
are alive,'' and \.{C$k$} accounts for the other four neighbors.
Codes \.D, \.E, \.F, and~\.G refer to pairs of neighbors.
Thus, for instance, \.{10a11C2} means that at least two of the
last four neighbors of cell $(10,11)$ are alive.

Those auxiliary variables receive values by means of up to 77 clauses per cell.
For example, if $u$ and~$v$ are the neighbors of cell~$z$ that correspond
to a pairing of type~\.D, there are six clauses
$$\bar u d_1,\quad
  \bar v d_1,\quad

Example code from Wikipedia:

% This file is part of CWEB.
 % This program by Silvio Levy and Donald E. Knuth
 % is based on a program by Knuth.
 % It is distributed WITHOUT ANY WARRANTY, express or implied.
 % Version 3.64 --- January 2002
 
 % Copyright (C) 1987,1990,1993,2000 Silvio Levy and Donald E. Knuth 
 
 % Permission is granted to make and distribute verbatim copies of this
 % document provided that the copyright notice and this permission notice
 % are preserved on all copies.
 
 % Permission is granted to copy and distribute modified versions of this
 % document under the conditions for verbatim copying, provided that the
 % entire resulting derived work is given a different name and distributed
 % under the terms of a permission notice identical to this one.
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Last updated February 11th, 2019