fixed typo in 3c of ps1
added unit 3 materials; still modifying them...
added unit3 materials plus
the png files for unit2 latex file and niceR.sh for unit2
added unit4 materials;
unit4 still being modified;
finalized unit3
more updated to unit4
updated to sec7 of unit 4
finalized unit4 except sec 8 (regexp)
Commititng.Merge remote-tracking branch 'upstream/master'