graphviz-java/src/main/java/guru/nidi/graphviz/service/CommandRunner.java
static Stream<Path> which(String program) {
        return which(program, Optional.ofNullable(System.getenv("PATH")).orElse(""));
    }
static Stream<Path> which(String program) {
        return which(program, Optional.ofNullable(System.getenv("PATH")).orElse(""));
    }
Cross-platform way of finding an executable in the $PATH.