.foo {
  position: absolute; @debug   10px + 20px  ;
  display: inline-block;
  @error   '  Sorry,   but `#{$variable}` is not a valid value for $variable.  ';
}


@debug 10px    +    20px;  $wrn:20px;


@warn
"   The     `border-radius()`
mixin will be deprecated in version 2.0.   ";


.bar {
  position: absolute;
  $wrn: 20px;  @warn '   #{$wrn}   '; display: inline-block;
}

/* http://www.sitepoint.com/using-sasss-error-warn-and-debug-directives/ */
@if unit($input) == 'px' {
  @warn
  "Please remember to use rem values in the `do-math()` function.";
  // do px math and @return a value;
}