Add an explanatory comment for making ranges unique

packages/sol-cov/src/coverage_manager.ts

@@ -42,6 +42,7 @@ export class CoverageManager {
         const coverageEntriesDescription = collectCoverageEntries(contractData.sourceCodes[fileIndex], fileName);
         let sourceRanges = _.map(coveredPcs, coveredPc => pcToSourceRange[coveredPc]);
         sourceRanges = _.compact(sourceRanges); // Some PC's don't map to a source range and we just ignore them.
+        // By default lodash does a shallow object comparasion. We JSON.stringify them and compare as strings.
         sourceRanges = _.uniqBy(sourceRanges, s => JSON.stringify(s)); // We don't care if one PC was covered multiple times within a single transaction
         sourceRanges = _.filter(sourceRanges, sourceRange => sourceRange.fileName === fileName);
         const branchCoverage: BranchCoverage = {};