In the SPL processStep model (and generally the RIM Act model), all steps may have sub-steps. In fact, as a "rule of infinite decomposability", every step can be decomposed into sub-steps as some more detail may always be required for some special use cases. There are two kinds of decomposition of steps into substeps, one is sequential, such as the Separate step being decomposed of four sub-steps: (1) addition of chloroform to a product mix of a previous reaction, (2) a time of stirring, (3) a time of waiting to allow the separation to occur, and finally (4) the transfer of the product phase into the receiving vessel. Defining these as a composite step with sub-steps is useful, because we can give a repeatNumber to the composite step, for example, to perform that separation step multiple times. In such a case, the separation may also include an evaporation step to remove the product phase solvent.