public <T> T old(T expression) {
    throw new InvalidSpecException("old() can only be used in a 'then' block");
  }
Used in a then-block to access an expression's value at the time just
before the previous where-block was entered.

@param expression an arbitrary expression, except that it may not
reference variables defined in the then-block
@param <T> the expression's type
@return the expression's value at the time the previous where-block was
entered
spock-core/src/main/java/spock/lang/Specification.java