      <p>This SRFI defines an interface to hash tables,
      which are widely recognized as a fundamental data structure for a wide
      variety of applications. A hash table is a data structure that:
      <ul>
        <li>Is disjoint from all other types.
        <li>
          Provides a mapping from objects known as
          <em>keys</em> to corresponding objects known as
          <em>values</em>.
          <ul>
            <li>Keys may be any Scheme objects in some
            kinds of hash tables, but are restricted in other kinds.
            <li>Values may be any Scheme objects.
          </ul>
        <li>Has no intrinsic order for the key-value
        <em>associations</em> it contains.
        <li>Provides an <em>equality predicate</em>
        which defines when a proposed key is the same as an existing key. No
        table may contain more than one value for a given key.
        <li>Provides a <em>hash function</em> which maps
        a candidate key into a non-negative exact integer.
        <li>Supports mutation as the primary means of
        setting the contents of a table.
        <li>Provides key lookup and destructive update
        in (expected) amortized constant time, provided a satisfactory hash
        function is available.
        <li>Does not guarantee that whole-table
        operations work in the presence of concurrent mutation of the whole
        hash table (values may be safely mutated).
      </ul>