Enhance grammar to support attribute mapping from sources to target #82

All 45 tests pass in 1m 27s

    3 files ±0   12 suites ±0 1m 27s ⏱️ +3s
  45 tests ±0   45 ✔️ ±0 0 💤 ±0 0 ❌ ±0
138 runs ±0 138 ✔️ ±0 0 💤 ±0 0 ❌ ±0

Results for commit f4dd129. ± Comparison against earlier commit 43c0d44.