+.\" Copyright (c) 1980 Regents of the University of California.
+.\" All rights reserved. The Berkeley software License Agreement
+.\" specifies the terms and conditions for redistribution.
+.\"
+.\" @(#)chb.n 4.1 (Berkeley) %G%
+.\"
+." @(#)chb.n 34.2 7/9/81
+.Ap 2 Special\ Symbols
+.pp
+The values of these symbols have a predefined meaning.
+Some values are counters
+while others are simply flags whose value the user can change to affect
+the operation of lisp system.
+In all cases, only the value cell of the symbol is important, the function
+cell is not.
+The value of some of the symbols (like \fBER%misc\fP)
+are functions - what this means is that the value cell of those symbols
+either contains a lambda expression,
+a binary object,
+or symbol with a function binding.
+.pp
+The values of the special symbols are:
+.in .5i
+.de Sa
+.sp 1v
+.ti -.5i
+\fB\\$1\fP\ \-\ \\
+..
+.Sa $gccount$
+The number of garbage collections which have occurred.
+.Sa $gcprint
+If bound to a non nil value, then after each garbage collection and
+subsequent storage allocation a summary of storage allocation will
+be printed.
+.Sa $ldprint
+If bound to a non nil value, then during each
+.i fasl
+or
+.i cfasl
+a diagnostic message will be printed.
+.Sa ER%all
+The function which is the error handler for all errors (see \(sc10)
+.Sa ER%brk
+The function which is the handler for the
+error signal generated by the evaluation of the
+.i break
+function (see \(sc10).
+.Sa ER%err
+The function which is the handler for the error
+signal generated by the evaluation of the
+.i err
+function (see \(sc10).
+.Sa ER%misc
+The function which is the handler of the error
+signal generated by one of the unclassified errors (see \(sc10).
+Most errors are unclassified at this point.
+.Sa ER%tpl
+The function which is the handler to be called
+when an error has occurred which
+has not been handled (see \(sc10).
+.Sa ER%undef
+The function which is the handler for the
+error signal generated when a call to an undefined function
+is made.
+.Sa ^w
+When bound to a non nil value this will prevent output to the standard
+output port (poport) from reaching the standard output (usually a terminal).
+Note that ^w is a two character symbol and should not be confused
+with ^W which is how we would denote control-w.
+The value of ^w is checked when the standard output buffer is flushed
+which occurs after a
+.i terpr ,
+.i drain
+or when the buffer overflows.
+This is most useful in conjunction with ptport described below.
+System error handlers rebind ^w to nil when they are invoked to assure
+that error messages are not lost.
+(This was introduced for Maclisp compatibility).
+.Sa defmacro-for-compiling
+The has an effect during compilation.
+If non-nil it causes macros defined by defmacro to be
+compiled and included in the
+object file.
+.Sa environment
+The UNIX environment in assoc list form.
+.Sa errlist
+When a
+.i reset
+is done, the value of errlist is saved away and control is thrown to
+the top level.
+\fIEval\fP
+is then mapped over the saved away
+value of this list.
+.Sa errport
+This port is initially bound to the standard error file.
+.Sa gcdisable
+If non nil, then garbage collections will not be done automatically when
+a collectable data type runs out.
+.Sa ibase
+This is the input radix used by the lisp reader.
+It may be either eight or ten.
+Numbers followed by a decimal point are assumed to be decimal regardless
+of what ibase is.
+.Sa linel
+The line length used by the pretty printer, pp.
+This should be used by
+.i print
+but it is not at this time.
+.Sa nil
+This symbol represents the null list and thus can be written ().
+Its value is always nil.
+Any attempt to change the value will result in an error.
+.Sa piport
+Initially bound to the standard input (usually the keyboard).
+A read with no arguments reads from piport.
+.Sa poport
+Initially bound to the standard output (usually the terminal console).
+A print with no second argument writes to poport.
+See also: ^w and ptport.
+.Sa prinlength
+If this is a positive fixnum, then the \fIprint\fP function will print
+no more than prinlength elements of a list or hunk and further elements
+abbreviated as `...'.
+The initial value of prinlength is nil.
+.Sa prinlevel
+If this is a positive fixnum, then the \fIprint\fP function will print
+only prinlevel levels of nested lists or hunks.
+Lists below this level will be abbreviated by `&' and hunks below this
+level will be abbreviated by a `%'.
+The initial value of prinlevel is nil.
+.Sa ptport
+Initially bound to nil.
+If bound to a port, then all output sent to the standard output will
+also be sent to this port as long as this port is not also
+the standard output (as this would cause a loop).
+Note that ptport will not get a copy of whatever is sent to poport
+if poport is not bound to the standard output.
+.Sa readtable
+The value of this is the current readtable.
+It is an array but you should NOT try to change the value of the elements
+of the array using the array functions.
+This is because the readtable is an array of bytes and the smallest
+unit the array functions work with is a full word (4 bytes).
+You can use
+.i setsyntax
+to change the values and
+.i "(status syntax ...)"
+to read the values.
+.Sa t
+This symbol always has the value t.
+It is possible to change the value of this symbol for short
+periods of time but you are strongly advised against it.
+.Sa top-level
+In a lisp system without /usr/lib/lisp/toplevel.l loaded, after a
+.i reset
+is done, the lisp system will
+.i funcall
+the value of top-level if it is
+non nil.
+This provides a way for the user to introduce his own top level interpreter.
+When /usr/lib/lisp/toplevel.l is loaded, it sets top-level to franz-top-level
+and changes the
+.i reset
+function so that once franz-top-level starts, it cannot
+be replaced by changing top-level.
+Franz-top-level does provide a way of changing
+the top level however, and that is
+through user-top-level.
+.Sa user-top-level
+If this is bound then after a
+.i reset ,
+the value of this variable will be
+.i funcall ed.
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