Calculates the positional distribution of oligonucleotides in a set of sequences, and detects those which significantly discard from a homogeneous distribution. Warning: this program is useful for large data sets (some hundreds or thousands of sequences), pre-aligned on some signal (e.g. start codon). Reference: van Helden, J., del Olmo, M. and Perez-Ortin, J. E. (2000). Statistical analysis of yeast genomic downstream sequences reveals putative polyadenylation signals. Nucleic Acids Res 28, 1000-10.