/func {
/var1 exch def
/var2 exch def
... process ...
} def

(var2)(var1)func

(Stack is no empty) {/var2 exch def}{/var2 (default) def} ifelse

/rotate { % angle [matrix]  rotate  -
    dup type /arraytype ne
    { true exch matrix } % no array: create array, concat later
    { false 3 1 roll } % array: do not create, do not concat later
    ifelse % bool angle matrix
    dup 0 2 index cos put % bool angle matrix
    dup 1 2 index sin put % bool angle matrix
    dup 2 2 index sin neg put % bool angle matrix
    dup 3 4 3 roll cos put % bool matrix
    exch { concat } if % [matrix]
} def

/func { %<<params>>
%begin currentdict /var2 2 copy known not {(default) put}{pop pop} ifelse
<</var2(default)>> copy begin
% ... process ...
end } def

<<
/var1 (var1)
/var2 (var2)
>> func