init research

dataframe/plugins/kotlin-dataframe/testData/box/csDsl/single.kt

@@ -0,0 +1,18 @@
+import org.jetbrains.kotlinx.dataframe.*
+import org.jetbrains.kotlinx.dataframe.annotations.*
+import org.jetbrains.kotlinx.dataframe.api.*
+import org.jetbrains.kotlinx.dataframe.io.*
+
+fun box(): String {
+    compareSchemas(
+        df.select { name },
+        df.select { name }.select { single() },
+        df.select { nameStartsWith("name").single() },
+    )
+
+    compareSchemas(
+        df.select { name.firstName },
+        df.remove { name.lastName }.select { name.singleCol() },
+    )
+    return "OK"
+}