Make sure we have 'need' elements from current position {@link #p p}. Last valid
{@code p} index is {@code tokens.length-1}. {@code p+need-1} is the tokens index 'need' elements
ahead. If we need 1 element, {@code (p+1-1)==p} must be less than {@code tokens.length}.
Make sure we have 'need' elements from current position {@link #p p}. Last valid {@code p} index is {@code tokens.length-1}. {@code p+need-1} is the tokens index 'need' elements ahead. If we need 1 element, {@code (p+1-1)==p} must be less than {@code tokens.length}.