code: @Override
  	public String onVariable(String variable) {
  		switch(variable) {
  		case "USERHOME":
  			return System.getProperty("user.home");
  		case "WINDIR":
  			return System.getenv("windir");
  		case "SYSTEMROOT":
  			return System.getenv("SystemRoot");
  		case "COMMONPROGRAMFILES":
  			return System.getenv("CommonProgramFiles");
  		case "PROGRAMFILES":
  			return System.getenv("ProgramFiles");
  		case "TEMP":
  			return System.getenv("TEMP");
  		case "SYSTEMDRIVE":
  			return System.getenv("SystemDrive");
  		case "USERPROFILE":
  			return System.getenv("USERPROFILE");
  		case "APPDATA":
  			return System.getenv("APPDATA");
  		}
  		return null;
  	}
docs: Returns the value of the system property or environment variable corresponding
  to the given variable name, {@code null} if not matching any of the known
  variables.
  
  @see org.dihedron.core.variables.ValueProvider#onVariable(java.lang.String)