HOW'TO ANALYZE line:
    SHARE followers, starters, enders
    IF #line>2:
        PUT 1 th'of line, 2 th'of line IN c1, c2
        INSERT c1, c2 IN starters
        INSERT line@#line IN enders
        FOR c3 IN line@3:
            IF (c1, c2) not'in keys followers:
                PUT {} IN followers[c1,c2]
            INSERT c3 IN followers[c1, c2]
            PUT c2, c3 IN c1, c2