private String getDescription(Statement stat) {
    ConstantExpression constExpr = AstUtil.getExpression(stat, ConstantExpression.class);
    return constExpr == null || !(constExpr.getValue() instanceof String) ?
        null : (String)constExpr.getValue();
  }
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