I am new to web development processes. I was setting gulp, postcss and everything. So I decided to test if it worked. Tried gulp-autoprefixer - it worked, then tried to use postcss with autoprefixer passed as parameter - it worked as well. I was reading through articles on the internet and decided to test a custom processor:

var opacity = function(css) {
  css.eachDecl(function(decl, i) {
    if (decl.prop === 'opacity') {
      decl.parent.insertAfter(i, {
        prop: '-ms-filter',
        value: '"progid:DXImageTransform.Microsoft.Alpha(Opacity=' + (parseFloat(decl.value) * 100) + ')"'
      });
    }
  });
};

gulp.task('styles', function() {
  return gulp.src('app/styles/main.scss')
    .pipe(sass())
    .pipe(autoprefixer({browsers: ['last 3 versions']}))
    .pipe(postcss([opacity]))
    .pipe(gulp.dest('dist/styles'));
});

It started throwing all kinds of errors. At the start I thought I am being retarded and there is a mistake somewhere on the website I am reading, or maybe I made some mistake.
So I started reading this postcss github and found same example with opacity processor. Well, I tried example from this github and it continued to throw errors.
After ~2 hours of reading articles and trying to find a solution for this problem I randomly found out that there are no eachDecl function in postcss' API but I found walkDecls function there.

So I thought that this one might work because they updated API but forgot to update examples. And walkDecls did work.

Might be a good idea to update wiki to not send people on a wild-goose chase.