Is your feature request related to a problem? Please describe.
See #803

See example of calculatedExpression and variable: http://build.fhir.org/ig/HL7/sdc/examples.html#using-calculatedexpression-and-variable

@maimoonak provided this example where we want to take age OR birthdate as input and calculate the other from one which was input: https://github.com/google/android-fhir/blob/mssng-quest-feat/datacapturegallery/src/main/assets/sample_family_member_registration_calculatedexpression.json

Describe the solution you'd like
Probably use FHIRPath

Describe alternatives you've considered
NA

Additional context
NA

Would you like to work on the issue?
NA