add concat

Author: MarcoGorelli

spec/API_specification/dataframe_api/__init__.py

@@ -70,6 +70,9 @@ def concat(dataframes: Sequence[DataFrame]) -> DataFrame:
     The order in which the input DataFrames appear in
     the output is preserved (so long as the DataFrame implementation supports row
     ordering).
+
+    To be guaranteed to run across all implementations, :meth:`maybe_execute` should
+    be executed on all input dataframes at some point before calling this method.
     """
     ...
 

spec/design_topics/execution_model.md

@@ -66,6 +66,7 @@ Operations which require `DataFrame.maybe_execute` to have been called at some p
 point are:
 - `DataFrame.to_array`
 - `DataFrame.shape`
+- `concat`
 - calling `bool`, `int`, or `float` on a scalar 
 
 Note now `DataFrame.maybe_execute` is called only once, and as late as possible.