src/main/java/com/github/fge/grappa/parsers/BaseActions.java
public boolean push(final int down, final V value)
    {
        check();
        context.getValueStack().push(down, value);
        return true;
    }
public boolean push(final int down, final V value)
    {
        check();
        context.getValueStack().push(down, value);
        return true;
    }
Inserts the given value a given number of elements below the current top
of the value stack

@param down the number of elements to skip before inserting the value (0
being equivalent to {@code push(value)})
@param value the value
@return true

@throws IllegalArgumentException the stack does not contain enough
elements to perform this operation