This function is used to extract the abstracts from the large corpus excluding the years and under a given theme. Its output is used in other functions like BWI and genes_BWI
previousabs_fn(yrs_to_exclude, theme, parentabs)
It returns an S4 object containing the abstracts of the given year.
yrs_to_exclude
is abstracts for the list of years we want to exclude from the corpus
theme
is a character value specifying the themes for the Abstracts.
parentabs
an S4 object containing the Abstracts.
S.Ramachandran
currentabs_fn