An inference rule that specifies paths has the following syntax:
An inference rule applies to a dependency only if paths specified in the dependency exactly match the inference-rule paths. Specify the dependent's directory in frompath and the target's directory in topath; no spaces are allowed. Specify only one path for each extension. A path on one extension requires a path on the other. To specify the current directory, use either a period (.) or empty braces ( { } ). Macros can represent frompath and topath; they are invoked during preprocessing.