returns concordance text for input file.
concordance_file(filename, pattern, encoding = getOption("encoding"),
span = 5)file name
patterns of central words
filename's encoding
how many character will be produced around input pattern
Church, K. W. and Mercer, R. L. (1993). Introduction to the special issue on computational linguistics using large corpora. Computational Linguistics, 19(1):1-24.