in jade, arguments passed to a `mixin` have unevaluated variables

when an argument to a mixin that contains a variable, it is apparently not evaluated.

for example,

mixin js( out, lcl )
  if suffix == 'out'
    script( src= net )
  else
    script( src= lcl )

mixin js( "http://cdnjs.cloudflare.com/ajax/libs/headjs/0.99/head.load.min.js",   "http://trailsandtribulations.#{suffix}/scripts/head.load.min.js" );

yields

<script src="http://trailsandtribulations.#{suffix}/scripts/head.load.min.js">

But the variable suffix works normally

for example,

script( src="http://trailsandtribulations.#{suffix}/scripts/head.load.min.js" )

yields

<script src="http://trailsandtribulations.nat/scripts/head.load.min.js">

Am I missing something here?