An intermediate representation of sequences of instructions for a stacked based computer is a code graph using a numbering method on the nodes of the graph, along with a set of relations among the nodes, to determine, in a single pass, the independence of each node or sub-graph represented by the node. The numbering is a post-order that directly, by numerical comparison defines the relevant hierarchical relationships among sub-graphs. The sub-graph of a particular node may have one or more alias nodes that refers to target nodes, a target node being a node representing an argument which is the result of a previous program instruction. For a subgraph to be considered independent, any aliases generated by nodes within the subgraph must themselves be contained in it, and conversely, any aliases in the subgraph must have been generated by nodes also within it.