Trying to re-summarize this accurately as an Octave feature request.

The 'taylor' function in the symbolic package is actually a method of the '@sym' class. If a user calls 'taylor(x)' on a symbolic argument, it works, but if a user calls 'help taylor' or 'taylor(x)' where 'x' is a numeric argument, Octave only reports that the function is not defined.

This bug is solely about adding a better error message to indicate that users should load the appropriate package.

The thing that needs to happen is a change to Octave, not individual function files.

This is not the kind of change that needs or even should be made to individual functions.  It's a generic problem with the way the Octave looks up functions compared to the way that Matlab appears to do it now, even for built-in functions.

when calling taylor from the symbolic package with an incorrect argument type, it produces an error message that makes it seem more like the function isn't loaded than that it just isn't being called correctly. Don't know if this also applies to other functions/packages in Octave / Octave Forge:

>> b = [1:10];
>> taylor(sin(b))
error: 'taylor' undefined near line 1 column 1
>> pkg load symbolic
>> a = sym('a');
OctSymPy v2.2.2: this is free software without warranty, see source.
Initializing communication with SymPy using a popen2() pipe.
Detected Windows: using "winwrapy.bat" to workaround Octave bug #43036
Some output from the Python subprocess (pid 14012) might appear next.

OctSymPy: Communication established.  SymPy v0.7.6.
Python 2.7.9 (default, Dec 10 2014, 12:24:55) [MSC v.1500 32 bit (Intel)]
>> taylor(sin(a))
ans = (sym)

    5    3
   a    a
  --- - -- + a
  120   6

>> taylor(sin(b))
error: 'taylor' undefined near line 1 column 1
>>

run in Octave 4.0.0 on Windows , symbolic package 2.2.2

Also, FYI calling taylor in Matlab with a non-symbolic argument provides the following feedback:

Undefined function 'taylor' for input arguments of type 'double'.

Seems like Octave should give a type error, too, here.

related to this help discussion thread:
http://octave.1599824.n4.nabble.com/Taylor-expansion-does-not-expand-well-tp4673968.html