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+=pod
+
+=head1 NAME
+
+BIO_push, BIO_pop - add and remove BIOs from a chain.
+
+=head1 SYNOPSIS
+
+ #include <openssl/bio.h>
+
+ BIO *	BIO_push(BIO *b,BIO *append);
+ BIO *	BIO_pop(BIO *b);
+
+=head1 DESCRIPTION
+
+The BIO_push() function appends the BIO B<append> to B<b>, it returns
+B<b>.
+
+BIO_pop() removes the BIO B<b> from a chain and returns the next BIO
+in the chain, or NULL if there is no next BIO. The removed BIO then
+becomes a single BIO with no association with the original chain,
+it can thus be freed or attached to a different chain.
+
+=head1 NOTES
+
+The names of these functions are perhaps a little misleading. BIO_push()
+joins two BIO chains whereas BIO_pop() deletes a single BIO from a chain,
+the deleted BIO does not need to be at the end of a chain.
+
+The process of calling BIO_push() and BIO_pop() on a BIO may have additional
+consequences (a control call is made to the affected BIOs) any effects will
+be noted in the descriptions of individual BIOs.
+
+=head1 EXAMPLES
+
+For these examples suppose B<md1> and B<md2> are digest BIOs, B<b64> is
+a base64 BIO and B<f> is a file BIO.
+
+If the call:
+
+ BIO_push(b64, f);
+
+is made then the new chain will be B<b64-chain>. After making the calls
+
+ BIO_push(md2, b64);
+ BIO_push(md1, md2);
+
+the new chain is B<md1-md2-b64-f>. Data written to B<md1> will be digested
+by B<md1> and B<md2>, B<base64> encoded and written to B<f>.
+
+It should be noted that reading causes data to pass in the reverse
+direction, that is data is read from B<f>, base64 B<decoded> and digested
+by B<md1> and B<md2>. If the call:
+
+ BIO_pop(md2);
+
+The call will return B<b64> and the new chain will be B<md1-b64-f> data can
+be written to B<md1> as before.
+
+=head1 RETURN VALUES
+
+BIO_push() returns the end of the chain, B<b>.
+
+BIO_pop() returns the next BIO in the chain, or NULL if there is no next
+BIO.
+
+=head1 SEE ALSO
+
+TBA