diff --git a/Cargo.lock b/Cargo.lock
index b090ed7949..db3ff5619e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -55,6 +55,16 @@ name = "fixedbitset"
 version = "0.1.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
+[[package]]
+name = "im-rc"
+version = "13.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "rustc_version 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
+ "sized-chunks 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "typenum 1.10.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
 [[package]]
 name = "log"
 version = "0.4.6"
@@ -102,12 +112,47 @@ version = "0.1.0"
 dependencies = [
  "combine 3.8.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "dogged 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "im-rc 13.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "maplit 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "petgraph 0.4.13 (registry+https://github.com/rust-lang/crates.io-index)",
  "pretty_assertions 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "rustc_version"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "semver 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "semver"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "semver-parser 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "semver-parser"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "sized-chunks"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "typenum 1.10.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "typenum"
+version = "1.10.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "unreachable"
 version = "1.0.0"
@@ -150,12 +195,18 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 "checksum dogged 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "2638df109789fe360f0d9998c5438dd19a36678aaf845e46f285b688b1a1657a"
 "checksum either 1.5.1 (registry+https://github.com/rust-lang/crates.io-index)" = "c67353c641dc847124ea1902d69bd753dee9bb3beff9aa3662ecf86c971d1fac"
 "checksum fixedbitset 0.1.9 (registry+https://github.com/rust-lang/crates.io-index)" = "86d4de0081402f5e88cdac65c8dcdcc73118c1a7a465e2a05f0da05843a8ea33"
+"checksum im-rc 13.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "0a0197597d095c0d11107975d3175173f810ee572c2501ff4de64f4f3f119806"
 "checksum log 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)" = "c84ec4b527950aa83a329754b01dbe3f58361d1c5efacd1f6d68c494d08a17c6"
 "checksum maplit 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)" = "08cbb6b4fef96b6d77bfc40ec491b1690c779e77b05cd9f07f787ed376fd4c43"
 "checksum memchr 2.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "2efc7bc57c883d4a4d6e3246905283d8dae951bb3bd32f49d6ef297f546e1c39"
 "checksum ordermap 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)" = "a86ed3f5f244b372d6b1a00b72ef7f8876d0bc6a78a4c9985c53614041512063"
 "checksum petgraph 0.4.13 (registry+https://github.com/rust-lang/crates.io-index)" = "9c3659d1ee90221741f65dd128d9998311b0e40c5d3c23a62445938214abce4f"
 "checksum pretty_assertions 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)" = "3a029430f0d744bc3d15dd474d591bed2402b645d024583082b9f63bb936dac6"
+"checksum rustc_version 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)" = "138e3e0acb6c9fb258b19b67cb8abd63c00679d2851805ea151465464fe9030a"
+"checksum semver 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)" = "1d7eb9ef2c18661902cc47e535f9bc51b78acd254da71d375c2f6720d9a40403"
+"checksum semver-parser 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)" = "388a1df253eca08550bef6c72392cfe7c30914bf41df5269b68cbd6ff8f570a3"
+"checksum sized-chunks 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "a2a2eb3fe454976eefb479f78f9b394d34d661b647c6326a3a6e66f68bb12c26"
+"checksum typenum 1.10.0 (registry+https://github.com/rust-lang/crates.io-index)" = "612d636f949607bdf9b123b4a6f6d966dedf3ff669f7f045890d3a4a73948169"
 "checksum unreachable 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "382810877fe448991dfc7f0dd6e3ae5d58088fd0ea5e35189655f84e6814fa56"
 "checksum void 1.0.2 (registry+https://github.com/rust-lang/crates.io-index)" = "6a02e4885ed3bc0f2de90ea6dd45ebcbb66dacffe03547fadbb0eeae2770887d"
 "checksum winapi 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)" = "92c1eb33641e276cfa214a0522acad57be5c56b10cb348b3c5117db75f3ac4b0"
diff --git a/Cargo.toml b/Cargo.toml
index ef6a221640..0a4232714b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -8,6 +8,7 @@ dogged = { version = "0.2.0", optional = true }
 log = "0.4"
 petgraph = { version = "0.4.5", optional = true }
 combine = "3.8.1"
+im-rc = "13.0.0"
 
 [dev-dependencies]
 pretty_assertions = "0.5.1"
diff --git a/src/lib.rs b/src/lib.rs
index fcf4fbc764..139cfdb674 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -9,4 +9,6 @@ pub mod parse_state;
 #[cfg(feature = "persistent")]
 extern crate dogged;
 
+extern crate im_rc;
+
 #[macro_use] extern crate combine;