core/src/main/java/org/trimou/engine/MustacheEngineBuilder.java
public MustacheEngineBuilder registerHelper(String name, Helper helper,
            boolean overwrite) {
        checkArgumentsNotNull(name, helper);
        checkNotBuilt();
        if (!overwrite && helpers.containsKey(name)) {
            throw new IllegalArgumentException(
                    "A helper with this name is already registered: " + name);
        }
        helpers.put(name, helper);
        return this;
    }
public MustacheEngineBuilder registerHelper(String name, Helper helper,
            boolean overwrite) {
        checkArgumentsNotNull(name, helper);
        checkNotBuilt();
        if (!overwrite && helpers.containsKey(name)) {
            throw new IllegalArgumentException(
                    "A helper with this name is already registered: " + name);
        }
        helpers.put(name, helper);
        return this;
    }
Each helper must be registered with a unique name. If there is a helper
registered with the same name and the param <code>overwrite</code> is
<code>true</code> the previous instance is replaced, otherwise an
{@link IllegalArgumentException} is thrown.

@param name
@param helper
@param overwrite
@return self