4I. assignments expressions (:=)
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Assignments (setting a variable to a value) exist as statement
and as expression. The only difference is that the statement
version has the form <varexp>:=<exp> and the expression <var>:=<exp>.
The latter has the value of <exp> as result.
Note that as <var>:= takes on an expression, you will often need
parentheses to force correct interpretation, like:

IF mem:=New(100)=NIL THEN error()

is interpreted like:

IF mem:=(New(100)=NIL) THEN error()

which is not what you mean: mem should be a pointer, not a boolean.
but you should write:

IF (mem:=New(100))=NIL THEN error()

it's a good habit to parenthesize any assignment expression that is
part of another one, if not already disambiguated by other constructs
such as "bla(a:=1)", "b:=a:=1" etc.