The grammar prepphrase.grm recognizes one-word prepositions and some multi-word prepositions as being of syntactic type PREP.  More prepositions, especially multi-word ones, will have to be added to it.  The module also recognizes a PREP followed by a noun phrase (NP) as being a prepositional phrase (PREPPHRASE).   PREPPHRASE is used in the module postdescrip.grm .  The largest diagram in this module is hard to follow when it is displayed with ASDEditor.  So it is helpful to display only selected component diagrams.  These can be displayed by selecting words and instances in the upper-left panes of an ASDEditor window.  Diagrams of particular interest which can be displayed that way include ones involving the instances (PREP 1), (PREP2 1), and (PREP-P 1).  Selected components can also be removed from the display by using the View>Hide>selectedComponent(s) menu selection at the top of the window, or the Hide>selectedComponent(s) selection from the pop-up menu in the bottom pane.  For example, if the entire network is displayed, the largest connected diagram in it can be removed from the display by selecting the node (among 1) and then hiding that selected component -- that is, the nodes that are connected to that node.
It is also helpful to examine the file prepphrase.grm as a simple text file, using a text file editor.