Changed unicode references from hex to names, see #6430

Replaced hexadecemal notation with named unicode references.
Names are fully interchangable with the hex codes being replaced.

Names obtained using unicodedata.name() function.
Example:

>>> import unicodedata
>>> print unicodedata.name(u'\u03c0')
GREEK SMALL LETTER PI
>>> u'\N{GREEK SMALL LETTER PI}'
u'\u03c0'
>>> u'\N{GREEK SMALL LETTER PI}' == u'\u03c0'
True
>>> u'\N{GREEK SMALL LETTER PI}' == u'\u03c1'
False
Changed unicode references from hex to names, see #6430

Replaced hexadecemal notation with named unicode references.
Names are fully interchangable with the hex codes being replaced.

Names obtained using unicodedata.name() function.
Example:

>>> import unicodedata
>>> print unicodedata.name(u'\u03c0')
GREEK SMALL LETTER PI
>>> u'\N{GREEK SMALL LETTER PI}'
u'\u03c0'
>>> u'\N{GREEK SMALL LETTER PI}' == u'\u03c0'
True
>>> u'\N{GREEK SMALL LETTER PI}' == u'\u03c1'
False
Corrected invalid unicode name

There is no greek letter 'I'
Don't use reduce in checkodesol()

It isn't avaiable in Python 3
Use set().union instead of set.union

This makes it work with no arguments. Thanks to @kjordahl for pointing this
out.
Add examples for summation with symbolic indices

fixes #8549
Use set().union instead of set.union

This makes it work with no arguments. Thanks to @kjordahl for pointing this
out.
cleanup other set.union cases
Merge pull request #67 from smichr/union

cleanup reduce/set.union usage
Merge pull request #8547 from emiller42/6430_unicode_names

#6430 unicode names
Merge pull request #8565 from asmeurer/reduce

Don't use reduce in checkodesol()
Merge pull request #8572 from skirpichev/8549-symb-ind-for-Sum

Add examples for summation with symbolic indices