Is the mapping purely a string or is there some defined structure as seems to be suggested in the example above? If the syntax is required/possible, we should state it and define it.

Existing Wording:

In the example above, the target is another FHIR IG, but in many cases, the target will not use FHIR. For this reason, the right-hand side of mapping statements is always a string, to allow the greatest flexibility. For the same reason, even though the target is FHIR in the example above, FSH cannot make any assumptions about how the individual target values work; thus the resource name “Patient” is included in the right-hand side values since this is how the mapping targets should be expressed in the SD.