to generate antlr4 java files and classes from grammar HM.g4

$ cd /usr/local/lib

$ sudo curl -O http://www.antlr.org/download/antlr-4.5-complete.jar

$ export CLASSPATH=".:/usr/local/lib/antlr-4.5-complete.jar:$CLASSPATH"

$ alias antlr4='java -jar /usr/local/lib/antlr-4.5-complete.jar'

$ antlr4 HM.g4

$ javac HM*.java