to the song structure of the input score 102 as shown at step 317 otherwise if the recombination mode is enabled the aggregation rules 134 may be employed to identify permissible song structures 136 or sequences of part types 161 1 the aggregation rules 136 identify a song structure such that the song structure i 136 is indicative of a sequence of part types 161 1 operable to provide an acceptable musical progression as shown at step 318 the recombiner 132 selects for each iteration a part variation 144 having a type 161 1 corresponding to the song structure 136 permitted by the aggregation rules 134 e g 520 540 other structures may be specified by the song structures 136 the corresponding types 161 1 are determinable from a mapping of types the mapping based on a logical musical progression defined by a predetermined song structure 520 540 as shown at step 319 the recombiner selects the next part type 161 1 by iterating through the sequence defined by the song structure 136 as shown at step 320 referring to fig9 while iterating searching for part variations corresponding to a duration 149 the recombiner 132 computes if the type 161 1 of part is adapted for inclusion based on the type check of step 312 part variations 144 of the part 114 such that each part variation has a different duration as shown at step 322 the recombiner determining a resizability attribute for each of the parts as.