add_to_database         Adds a single .txt file to an SQLite database
                        on the hard disk.
combine_query           Combine a database query with a cohort.
combine_query.aurum     Combine a CPRD aurum database query with a
                        cohort.
combine_query_boolean   Combine a database query with a cohort
                        returning a 0/1 vector depending on whether
                        each individual has a recorded code of
                        interest.
combine_query_boolean.aurum
                        Combine a CPRD aurum database query with a
                        cohort returning a 0/1 vector depending on
                        whether each individual has a recorded code of
                        interest.
connect_database        Open connection to SQLite database
cprd_extract            Adds all the .txt files in a directory, with
                        certain file names, to an SQLite database on
                        the hard disk.
create_directory_system
                        Create the appropriate directory system to be
                        able to run functions without specifying hard
                        filepaths
db_query                Query an RSQLite database.
delete_directory_system
                        Deletes directory system created by
                        'delete_directory_system'
extract_bmi             Extract most recent BMI score relative to an
                        index date.
extract_cholhdl_ratio   Extract most recent total
                        cholesterol/high-density lipoprotein ratio
                        score relative to an index date.
extract_cohort          Create cohort from patient files
extract_diabetes        Extract diabetes status prior to an index date.
extract_ho              Extract a 'history of' type variable
extract_practices       Combine practice files
extract_smoking         Extract smoking status prior to index date.
extract_test_data       Extract test data.
extract_test_data_var   Extract standard deviation of all test data
                        values over a specified time period relative to
                        an index date.
extract_test_recent     Extract test data.
extract_time_until      Extract a 'time until' type variable
extract_txt_char        Read in txt file with all colClasses =
                        "character"
extract_txt_cons        Read in raw .txt consultation file
extract_txt_death       Read in raw ONS death data file
extract_txt_drug        Read in raw .txt drugissue file
extract_txt_hes_primary
                        Read in raw HES primary diagnoses file
extract_txt_linkage     Read in linkage eligibility file
extract_txt_obs         Read in raw .txt observation file
extract_txt_pat         Read in raw .txt patient file
extract_txt_prac        Read in raw .txt practice file
extract_txt_prob        Read in raw .txt problem file
extract_txt_ref         Read in raw .txt referral file
implement_output        Internal function to implement saving extracted
                        variable to disk or returning into R workspace.
