I wrote in a previous contribution:
>
>The sign comes from the ALGOL60 programming language
>(1960) and means in say ``$i := j+1$'' that the variable i
>gets assigned the value in variable j plus 1.
>
>Since this is similar to definitions (theoretical) computer
>scientists have started to use it that way.
>
Cooke's remark gets explained by the fact that Wirth's Pascal
programming language is a modified version of ALGOL60. In fact,
Wirth was also one of the 10-15 scientists of the WorkGroup
WG2.1 of IFIP developing the standard ALGOL60 language and writing
the defining document "the ALGOL60 Report" in 1960.