Declared variable in style file

Question

I'm trying to read about .bst files from Tame the Beast. On page 39, there is this example for a multiplication function:

INTEGERS { a b }

FUNCTION {mult}
{
  ’a := %% we store the first value
  ’b := %% we store the second value

  b #0 < %% We remember the sign of b, and
    {#-1 #0 b - ’b :=} %% then consider its absolute value.
    {#1} %%
  if$
  ...

why are there b's that are not preceded by a '? Shouldn't all b's refer to the same b, the b that was declared inside INTEGERS, and should be preceded by a '?

Answer 1

The syntax of BibTeX is such that

#1 'a :=

is an assignment, with the variable named a being set to the value 1, while

a #1 =

is a comparison, with the value of the variable a inserted and compared to 1.

BibTeX is a stack language, so more generally 'a leaves the name of the variable on the stack while just a leaves the value on the stack.

Answer 2

The line {#-1 #0 b - ’b :=} means: set the value of the variable whose name is 'b to the value of obtained by the operation where the value of one of the operand is b (the value currently stored by the variable 'b.