diff --git a/README.md b/README.md
index 571fa20..78c3787 100644
--- a/README.md
+++ b/README.md
@@ -1,44 +1,219 @@
 ```javascript
 var assert = require('assert')
 var satisfies = require('spdx-satisfies')
+```
+
+This package exports a single function of two arguments:
+
+1.  an Object representing an SPDX expression
 
-assert(satisfies('MIT', 'MIT'))
+2.  an Array of Objects, each in the form of a leaf in an SPDX expression data structure
 
-assert(satisfies('MIT', '(ISC OR MIT)'))
-assert(satisfies('Zlib', '(ISC OR (MIT OR Zlib))'))
-assert(!satisfies('GPL-3.0', '(ISC OR MIT)'))
+```javascript
+assert(
+  satisfies(
+    {license: 'MIT'},
+    [{license: 'MIT'}]
+  )
+)
+```
 
-assert(satisfies('GPL-2.0', 'GPL-2.0+'))
-assert(satisfies('GPL-3.0', 'GPL-2.0+'))
-assert(satisfies('GPL-1.0+', 'GPL-2.0+'))
-assert(!satisfies('GPL-1.0', 'GPL-2.0+'))
-assert(satisfies('GPL-2.0-only', 'GPL-2.0-only'))
-assert(satisfies('GPL-3.0-only', 'GPL-2.0+'))
+The schema for SPDX expression data structures is the same returned by [spdx-expression-parse](https://www.npmjs.com/package/spdx-expression-parse).
+
+```javascript
+var parse = require('spdx-expression-parse')
+
+assert(satisfies(
+  parse('MIT'),
+  [parse('ISC'), parse('MIT')]
+))
+
+assert(satisfies(
+  {license: 'Zlib'},
+  [
+    {license: 'ISC'},
+    {license: 'MIT'},
+    {license: 'Zlib'}
+  ]
+))
 
 assert(!satisfies(
-  'GPL-2.0',
-  'GPL-2.0+ WITH Bison-exception-2.2'
-))
-
-assert(satisfies(
-  'GPL-3.0 WITH Bison-exception-2.2',
-  'GPL-2.0+ WITH Bison-exception-2.2'
-))
-
-assert(satisfies('(MIT OR GPL-2.0)', '(ISC OR MIT)'))
-assert(satisfies('(MIT AND GPL-2.0)', '(MIT AND GPL-2.0)'))
-assert(satisfies('MIT AND GPL-2.0 AND ISC', 'MIT AND GPL-2.0 AND ISC'))
-assert(satisfies('MIT AND GPL-2.0 AND ISC', 'ISC AND GPL-2.0 AND MIT'))
-assert(satisfies('(MIT OR GPL-2.0) AND ISC', 'MIT AND ISC'))
-assert(satisfies('MIT AND ISC', '(MIT OR GPL-2.0) AND ISC'))
-assert(satisfies('MIT AND ISC', '(MIT AND GPL-2.0) OR ISC'))
-assert(satisfies('(MIT OR Apache-2.0) AND (ISC OR GPL-2.0)', 'Apache-2.0 AND ISC'))
-assert(satisfies('(MIT OR Apache-2.0) AND (ISC OR GPL-2.0)', 'Apache-2.0 OR ISC'))
-assert(satisfies('(MIT AND GPL-2.0)', '(MIT OR GPL-2.0)'))
-assert(satisfies('(MIT AND GPL-2.0)', '(GPL-2.0 AND MIT)'))
-assert(satisfies('MIT', '(GPL-2.0 OR MIT) AND (MIT OR ISC)'))
-assert(satisfies('MIT AND ICU', '(MIT AND GPL-2.0) OR (ISC AND (Apache-2.0 OR ICU))'))
-assert(!satisfies('(MIT AND GPL-2.0)', '(ISC OR GPL-2.0)'))
-assert(!satisfies('MIT AND (GPL-2.0 OR ISC)', 'MIT'))
-assert(!satisfies('(MIT OR Apache-2.0) AND (ISC OR GPL-2.0)', 'MIT'))
+  {license: 'GPL-3.0'},
+  [
+    {license: 'ISC'},
+    {license: 'MIT'}
+  ]
+))
+
+
+assert(satisfies(
+  {license: 'GPL-2.0'},
+  [{license: 'GPL-2.0', plus: true}]
+))
+
+assert(satisfies(
+  {license: 'GPL-3.0'},
+  [{license: 'GPL-2.0', plus: true}]
+))
+
+assert(satisfies(
+  {license: 'GPL-1.0', plus: true},
+  [{license: 'GPL-2.0', plus: true}]
+))
+
+assert(!satisfies(
+  {license: 'GPL-1.0'},
+  [{license: 'GPL-2.0', plus: true}]
+))
+
+assert(satisfies(
+  {license: 'GPL-2.0-only'},
+  [{license: 'GPL-2.0-only'}]
+))
+
+assert(satisfies(
+  {license: 'GPL-3.0-only'},
+  [{license: 'GPL-2.0', plus: true}]
+))
+
+assert(!satisfies(
+  {license: 'GPL-2.0'},
+  [
+    {
+      license: 'GPL-2.0',
+      plus: true,
+      exception: 'Bison-exception-2.2'
+    }
+  ]
+))
+
+assert(satisfies(
+  {
+    license: 'GPL-3.0',
+    exception: 'Bison-exception-2.2'
+  },
+  [
+    {
+      license: 'GPL-2.0',
+      plus: true,
+      exception: 'Bison-exception-2.2'
+    }
+  ]
+))
+
+assert(satisfies(
+  // (MIT OR GPL-2.0)
+  {
+    left: {license: 'MIT'},
+    conjunction: 'or',
+    right: {license: 'GPL-2.0'}
+  },
+  [
+    {license: 'ISC'},
+    {license: 'MIT'}
+  ]
+))
+
+assert(satisfies(
+  // ((MIT OR Apache-2.0) AND (ISC OR GPL-2.0))
+  {
+    left: {
+      left: {license: 'MIT'},
+      conjunction: 'or',
+      right: {license: 'Apache-2.0'}
+    },
+    conjunction: 'and',
+    right: {
+      left: {license: 'ISC'},
+      conjunction: 'or',
+      right: {license: 'GPL-2.0'}
+    }
+  },
+  [
+    {license: 'Apache-2.0'},
+    {license: 'ISC'}
+  ]
+))
+
+assert(satisfies(
+  // (MIT AND GPL-2.0)
+  {
+    left: {license: 'MIT'},
+    conjunction: 'and',
+    right: {license: 'GPL-2.0'}
+  },
+  [
+    {license: 'MIT'},
+    {license: 'GPL-2.0'}
+  ]
+))
+
+assert(!satisfies(
+  // (MIT AND GPL-2.0)
+  {
+    left: {license: 'MIT'},
+    conjunction: 'and',
+    right: {license: 'GPL-2.0'}
+  },
+  [
+    {license: 'ISC'},
+    {license: 'GPL-2.0'}
+  ]
+))
+
+assert(!satisfies(
+  // (MIT AND (GPL-2.0 OR ISC))
+  {
+    left: {license: 'MIT'},
+    conjunction: 'and',
+    right: {
+      left: {license: 'GPL-2.0'},
+      conjunction: 'or',
+      right: {license: 'ISC'}
+    }
+  },
+  [{license: 'MIT'}]
+))
+
+assert(!satisfies(
+  // (MIT OR Apache-2.0) AND (ISC OR GPL-2.0)
+  {
+    left: {
+      left: {license: 'MIT'},
+      conjunction: 'or',
+      right: {license: 'Apache-2.0'}
+    },
+    conjunction: 'and',
+    right: {
+      left: {license: 'ISC'},
+      conjunction: 'or',
+      right: {license: 'GPL-2.0'}
+    }
+  },
+  [{license: 'MIT'}]
+))
+```
+
+The exported function does a few naive type checks on arguments.  Do not rely on it for rigorous validation.
+
+```javascript
+assert.throws(function () {
+  satisfies('MIT', [parse('MIT')])
+}, /first argument/)
+
+assert.throws(function () {
+  satisfies({invalid: 'AST'}, [parse('MIT')])
+}, /first argument/)
+
+assert.throws(function () {
+  satisfies(parse('MIT'), parse('MIT'))
+}, /second argument/)
+
+assert.throws(function () {
+  satisfies(parse('MIT'), parse('MIT'))
+}, /second argument/)
+
+assert.throws(function () {
+  satisfies(parse('MIT'), [{invalid: 'leaf'}])
+}, /second argument/)
 ```
diff --git a/index.js b/index.js
index f6c5a37..f3d22b9 100644
--- a/index.js
+++ b/index.js
@@ -1,7 +1,22 @@
 var compare = require('spdx-compare')
-var parse = require('spdx-expression-parse')
 var ranges = require('spdx-ranges')
 
+module.exports = function (first, second) {
+  if (!Array.isArray(second)) throw new Error('second argument must be an Array')
+  if (second.some(function (element) {
+    return !element.hasOwnProperty('license')
+  })) throw new Error('second argument must contain license expression AST leaves')
+  if (
+    !first.hasOwnProperty('license') &&
+    !first.hasOwnProperty('conjunction')
+  ) throw new Error('first argument must be a license expression AST')
+  var terms = normalizeGPLIdentifiers(first)
+  var whitelist = second.map(function (element) {
+    return normalizeGPLIdentifiers(element)
+  })
+  return recurse(terms, whitelist)
+}
+
 var rangesAreCompatible = function (first, second) {
   return (
     first.license === second.license ||
@@ -79,60 +94,17 @@ function endsWith (string, substring) {
   return string.indexOf(substring) === string.length - 1
 }
 
-function licenseString (e) {
-  if (e.hasOwnProperty('noassertion')) return 'NOASSERTION'
-  if (e.license) return `${e.license}${e.plus ? '+' : ''}${e.exception ? ` WITH ${e.exception}` : ''}`
-}
-
-// Expand the given expression into an equivalent array where each member is an array of licenses AND'd
-// together and the members are OR'd together. For example, `(MIT OR ISC) AND GPL-3.0` expands to
-// `[[GPL-3.0 AND MIT], [ISC AND MIT]]`. Note that within each array of licenses, the entries are
-// normalized (sorted) by license name.
-function expand (expression) {
-  return sort(Array.from(expandInner(expression)))
-}
-
-// Flatten the given expression into an array of all licenses mentioned in the expression.
-function flatten (expression) {
-  const expanded = Array.from(expandInner(expression))
-  const flattened = expanded.reduce(function (result, clause) {
-    return Object.assign(result, clause)
-  }, {})
-  return sort([flattened])[0]
-}
-
-function expandInner (expression) {
-  if (!expression.conjunction) return [{ [licenseString(expression)]: expression }]
-  if (expression.conjunction === 'or') return expandInner(expression.left).concat(expandInner(expression.right))
-  if (expression.conjunction === 'and') {
-    var left = expandInner(expression.left)
-    var right = expandInner(expression.right)
-    return left.reduce(function (result, l) {
-      right.forEach(function (r) { result.push(Object.assign({}, l, r)) })
-      return result
-    }, [])
+function recurse (terms, whitelist) {
+  var conjunction = terms.conjunction
+  if (!conjunction) {
+    return whitelist.some(function (whitelisted) {
+      return licensesAreCompatible(terms, whitelisted)
+    })
+  } else if (conjunction === 'or') {
+    return recurse(terms.left, whitelist) || recurse(terms.right, whitelist)
+  } else if (conjunction === 'and') {
+    return recurse(terms.left, whitelist) && recurse(terms.right, whitelist)
+  } else {
+    throw new Error('invalid terms')
   }
 }
-
-function sort (licenseList) {
-  var sortedLicenseLists = licenseList
-    .filter(function (e) { return Object.keys(e).length })
-    .map(function (e) { return Object.keys(e).sort() })
-  return sortedLicenseLists.map(function (list, i) {
-    return list.map(function (license) { return licenseList[i][license] })
-  })
-}
-
-function isANDCompatible (one, two) {
-  return one.every(function (o) {
-    return two.some(function (t) { return licensesAreCompatible(o, t) })
-  })
-}
-
-function satisfies (first, second) {
-  var one = expand(normalizeGPLIdentifiers(parse(first)))
-  var two = flatten(normalizeGPLIdentifiers(parse(second)))
-  return one.some(function (o) { return isANDCompatible(o, two) })
-}
-
-module.exports = satisfies
diff --git a/package.json b/package.json
index 31db82c..57f98d8 100644
--- a/package.json
+++ b/package.json
@@ -10,12 +10,12 @@
   ],
   "dependencies": {
     "spdx-compare": "^1.0.0",
-    "spdx-expression-parse": "^3.0.0",
     "spdx-ranges": "^2.0.0"
   },
   "devDependencies": {
     "defence-cli": "^2.0.1",
     "replace-require-self": "^1.1.1",
+    "spdx-expression-parse": "^3.0.0",
     "standard": "^11.0.0"
   },
   "keywords": [