Yank registered ref: http://qiita.com/sokada/items/2915d56c7e70c03ccb76

file0.txt

# To see what in the default register
:reg "

# To see what in the register 0
:reg 0 # yank register

file1.txt

# diw(delete inner word) will register in the default register.
# In order to put the yanked word, use `"0P`.
"0P

file2.txt

# Just to delete, use "_. 
# e.g. to delete a word
"_diw

file3.txt

# copy to register a
# <C-V> to change to visual mode and select line(s) or word(s) 
"ay # use a-z
# to paste it
"ap
# check it in register
:reg "a:
# Delete it in the register a
qaq
# check it again
:reg "a # should be nothing in register a
# help 
:h quote_alpha
# You can append to a register
"Ayy # to append to a line
# or use
:yank A 
# or append all lines which has def
:global/def/yank A

file4.txt

# <C-v> to change to visual mode and select lines
# to copy
"+y
# to paste
"+p