Setup to transition to lean 4.

.gitignore

@@ -1,3 +1,2 @@
-*.olean
-/_target
-/leanpkg.path
+/build
+/lake-packages/*

Bookshelf.lean

@@ -0,0 +1 @@
+def hello := "world"

Main.lean

@@ -0,0 +1,4 @@
+import «Bookshelf»
+
+def main : IO Unit :=
+  IO.println s!"Hello, {hello}!"

lakefile.lean

@@ -0,0 +1,15 @@
+import Lake
+open Lake DSL
+
+package «bookshelf» {
+  -- add package configuration options here
+}
+
+lean_lib «Bookshelf» {
+  -- add library configuration options here
+}
+
+@[default_target]
+lean_exe «bookshelf» {
+  root := `Main
+}

lean-toolchain

@@ -0,0 +1 @@
+leanprover/lean4:nightly-2023-02-10

leanpkg.toml

@@ -1,8 +0,0 @@
-[package]
-name = "bookshelf"
-version = "0.1"
-lean_version = "leanprover-community/lean:3.50.3"
-path = "src"
-
-[dependencies]
-mathlib = {git = "https://github.com/leanprover-community/mathlib", rev = "4c19a16e4b705bf135cf9a80ac18fcc99c438514"}