public Spec build(ClassNode clazz) {
    spec = new Spec(clazz);
    clazz.visitContents(this);
    return spec;
  }
Given the abstract syntax tree of a Groovy class representing a Spock
specification, builds an object model of the specification.

@author Peter Niederwieser
spock-core/src/main/java/org/spockframework/compiler/SpecParser.java