if(exists key) { 
  if defined key) { 
    if value supplied { 
      if(value is valid) { 
         if(value eq this) { 
            use this 
         } 
         elsif (value eq that) { 
            do that 
         } else { 
            yell
         } 
      } 
      else { 
         use default 
      }
   } 
   else if( value is required ) {
      do something
   }
   else
      do something else
   }
}
else {
   key not supplied...
}

##</code><code>##