The strangest of all of them is that two instructions (messages) as simple as the following ones make the compiler (oddly) complain:
System.console.writeLn( "Hola" );
They'll have to be substituted with similar instructions, in which the return value must be stored.
retVal = System.console.writeLn( "Hola" );
retVal = System.console.lf();
While it is true that this compilers (prowl and J--) have been excelent for showing Zero's capabilities, they have turned out to be not very stable, thus making the need to replace them in the medium-term.