There is no main method like there is in Java. The way you execute mantra code is simply to run the tool on a file full of statements. Here is a sample run:
$ java mantra.Tool <stdin> println("hi, mom"); hi, mom
Note that mantra takes in an entire program and translates it to Java, compiles it, and then runs it. Mantra is not an interpreter and cannot process line by line like Python or Ruby.