When performing wavelet transform it is necessary to extend your signal over array boundaries somehow (with exception of the case when Haar wavelet is used). All algorithms currently work via lifting scheme which supports two kinds of signal extension:

1. Extend the signal with zeros. The signal simply equals to zero out of array bounds and to content of the array inside array bounds. Call transform functions with :boundary-style :zero argument to choose this extension mode.
2. Mirror the signal from array boundaries, creating a continuous signal. This is a preferred extension mode, call transform functions with :boundary-style :mirror to use it.