The While programming language
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The While programming language is defined by the following
grammar.

  P  -->  input I; P
       |  Q; output E
       |  output E
  Q  -->  S
       |  S; Q
  I  -->  x  |  y  |  z  |  ...
  S  -->  accept
       |  reject
       |  skip
       |  I := E
       |  if B then S else S
       |  while B do S
       |  begin Q end
  E  -->  T  |  E + T  |  E - T
  T  -->  F  |  T * F  |  T / F
  F  -->  I  |  ( E )  |  0  |  1  |  2  |  ...
  B  -->  not B  |  E < E  |  E = E  |  E > E

Semantics:  Every While program mentions a finite number of
variables.  At the beginning of the program, all of those
variables are zero except for the input variable, whose value
is the input to the program.  The program accepts the input
if and only if it executes an accept statement or an output
statement, and rejects the input if and only if it executes
a reject statement; all three of those statements terminate
the program.  If the program never terminates, it is said to
loop.