Authors of accepted papers are encouraged to send a proposal for this special issue. Please take into account that papers submitted to the SI should be original and technically sound. Thus, the PRLetters submission which will be an extended version of your conference paper should include at least 30% new contribution (more experiments, proofs of theorems not included in the conference paper, more comparisons with other methods in the literature and so on). The title of the PRLetters paper should be different, the same Figures cannot be used and the common part of the conference paper and of the extended version cannot be verbatim the same.