@Hidden
    @SuppressWarnings( "unchecked" )
    public SELF self() {
        return (SELF) this;
    }
Useful base class for step definitions as it provides a {@link #self()} method
to create fluent interfaces.
<p>
Direct subclasses should provide introduction words (see {@link com.tngtech.jgiven.annotation.IntroWord}).
<p>
Typically one derives from one of the language-specific step definition classes, which
already provide a handful of useful introduction words.

@param <SELF> the type of the subclass
@see com.tngtech.jgiven.Stage
@see com.tngtech.jgiven.lang.de.Stufe
jgiven-core/src/main/java/com/tngtech/jgiven/base/StageBase.java