1 files changed, 7 insertions, 1 deletions

diff --git a/subprojects/frontend/src/index.tsx b/subprojects/frontend/src/index.tsx
index 2c0259bf..b42b8062 100644
--- a/subprojects/frontend/src/index.tsx
+++ b/subprojects/frontend/src/index.tsx
@@ -20,6 +20,12 @@ enum TaxStatus {
     child, student, adult, retired
 }
 
+int ageDifference(Person p, Person q) =
+    children(p, q), age(p, pAge), age(q, qAge) -> qAge - pAge.
+
+error invalidAgeDifference(Person p, Person q) <->
+    children(p, q), ageDifference(p, q) <= 0.
+
 % A child cannot have any dependents.
 pred invalidTaxStatus(Person p) <->
     taxStatus(p, child),
@@ -29,7 +35,7 @@ pred invalidTaxStatus(Person p) <->
     parent(p, q),
     !taxStatus(q, retired).
 
-indiv family.
+individual family.
 Family(family).
 members(family, anne).
 members(family, bob).