WIP on documentation of proposed routines.

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+'\"
+'\" Contribution from Don Porter, NIST, 2022. (not subject to US copyright)
+'\"
+'\" See the file "license.terms" for information on usage and redistribution
+'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
+'\"
+.TH Tcl_GetNumber 3 8.7 Tcl "Tcl Library Procedures"
+.so man.macros
+.BS
+.SH NAME
+Tcl_GetNumber, Tcl_GetNumberFromObj \- get numeric value from Tcl value
+.SH SYNOPSIS
+.nf
+\fB#include <tcl.h>\fR
+.sp
+\fB#include <tclTomMath.h>\fR
+.sp
+int
+\fBTcl_GetNumber\fR(\fIinterp, bytes, numBytes, clientDataPtr, typePtr\fR)
+.sp
+int
+\fBTcl_GetNumberFromObj\fR(\fIinterp, objPtr, clientDataPtr, typePtr\fR)
+.SH ARGUMENTS
+.AS Tcl_Interp clientDataPtr out
+.AP Tcl_Interp *interp out
+When non-NULL, error information is recorded here when the value is not
+in any of the numeric formats recognized by Tcl.
+.AP "const char" *bytes in
+Points to first byte of the string value to be examined.
+.AP size_t numBytes in
+The number of bytes, starting at \fIbytes\fR, that should be examined.
+If the value \fBTCL_INDEX_NONE\fR is provided, then all bytes should
+be examined until the first \fBNUL\fR byte terminates examination.
+.AP "void *" *clientDataPtr out
+Points to space where a pointer value may be written through which a numeric
+value is available to read.
+.AP int *typePtr out
+Points to space where a value may be written reporting what type of
+numeric storage is available to read.
+.AP Tcl_Obj *objPtr in
+A Tcl value to be examined.
+.BE
+.SH DESCRIPTION
+.PP
+These procedures enable callers to retrieve a numeric value from a
+Tcl value in a numeric format recognized by Tcl.
+.PP
+Tcl recognizes many values as numbers.  Several examples include:
+\fB"0"\fR, \fB" +1"\fR, \fB"-2 "\fR, \fB" 3 "\fR, \fB"0xdad1"\fR, \fB"0d09"\fR,
+\fB"1_000_000"\fR, \fB"4.0"\fR, \fB"1e-7"\fR, \fB"NaN"\fR, or \fB"Inf"\fR.
+When built-in Tcl commands act on these values as numbers, they are converted
+to a numeric representation for efficient handling in C code.  Tcl makes
+use of three C types to store these representations: \fBdouble\fR, 
+\fBTcl_WideInt\fR, and \fBmp_int\fR.  The \fBdouble\fR type is provided by the
+C language standard.  The \fBTcl_WideInt\fR type is declared in the Tcl
+header file, \fBtcl.h\fR, and is equivalent to the C standard type
+\fBlong long\fR on most platforms. The \fBmp_int\fR type is declared in the
+header file \fBtclTomMath.h\fR, and implemented by the LibTomMath
+multiple-precision integer library, included with Tcl.
+
+The routines \fBTcl_GetNumber\fR and \fBTcl_GetNumberFromObj\fR perform
+the same function.  They differ only in how the arguments present the Tcl
+value to be examined.  \fBTcl_GetNumber\fR accepts a counted string
+value in the arguments \fIbytes\fR and \fInumBytes\fR (or a
+\fBNUL\fR-terminated string value when \fInumBytes\fR is
+\fBTCL_INDEX_NONE\fR).  \fBTcl_GetNumberFromObj\fR accepts the Tcl value
+in \fIobjPtr\fR.
+
+Both routines examine the Tcl value and determine whether Tcl recognizes
+it as a number.  If not, both routines return \fBTCL_ERROR\fR and (when
+\fIinterp\fR is not NULL) record an error message and error code
+in \fIinterp\fR.
+
+If the examined value is recognized as a number, both routines return
+\fBTCL_OK\fR, and use the pointer arguments \fIclientDataPtr\fR
+and \fItypePtr\fR (which may not be NULL) to report information the
+caller can use to retrieve the numeric representation.  In all cases,
+both routines write to *\fIclientDataPtr\fR a pointer to the internal
+storage location where Tcl holds the numeric value.  When that
+internal storage is of type \fBdouble\fR
+
+
+
+.SH "SEE ALSO"
+Tcl_GetDouble, Tcl_GetDoubleFromObj, Tcl_GetWideIntFromObj
+.SH KEYWORDS
+double, double value, double type, integer, integer value, integer type,
+internal representation, value, value type, string representation