Enable resetting time signature in staff construction

core/src/main/kotlin/io/smnp/evaluation/evaluator/StaffEvaluator.kt

@@ -64,14 +64,19 @@ class StaffEvaluator : Evaluator() {
       val evaluatedSignature = calculateTimeSignature(evaluatedMeasure)
 
       currentSignature?.let {
-         if (evaluatedSignature != it) {
+         when {
-            val simplified = evaluatedSignature.simplified
+            it.numerator == 0 -> return
-            throw contextEvaluationException(
+            evaluatedSignature != it -> {
-               "Invalid time signature: expected ${it.numerator}/${it.denominator}, got ${simplified.numerator}/${simplified.denominator}",
+               val simplified = evaluatedSignature.simplified
-               measure.position,
+               throw contextEvaluationException(
-               environment
+                  "Invalid time signature: expected ${it.numerator}/${it.denominator}, got ${simplified.numerator}/${simplified.denominator}",
-            )
+                  measure.position,
+                  environment
+               )
+            }
          }
+
+         null
       }
    }