Consider the set of pairs "(ia,ib)" where "ia" is an index in "a"
    and "ib" is an index in "b".  Since the chains are circular, this
    set is a toroidal integer grid with periods "ma=NUMBER(a)"
    and "mb = NUMBER(b)", respectively.
    
    In this program, we only look for candidates that are defined
    by `perfect' matchings, i.e. matchings without half-steps.
    Such a matching is a segment of some diagonal line of the 
    "(ia,ib)" grid.
    
    Therefore, we enumerate the diagonals of the toroidal grid, and
    search for candidates within each diagonal in turn. The number of
    diagonals is "numDiagonal = GCD(ma,mb)", and each diagonal is a
    circular sequence of "diagSize = (ma*mb)/nDiags".  
    
    We number the diagonals from "[0..nDiags-1]"
    and their elements from [0..diagSize-1]", as follows:
    element index "r" of diagonal number "t" is 
    the pair "(ia,ib)" where "ia = r MOD ma" and "ib = (t-r) MOD mb".
    
    Within each diagonal "t", we compute a vector "d2" such that "d2[r]"
    is the distance squared between "a[ia]" and "b[ib]", where "ia,ib" 
    are the corresponding chain indices.
    
    We also compute a boolean vector "ok", such that "ok[r]" tells
    whether the the "minSteps+1" elements beginning with element "r"
    would be an acceptable candidate. Specifically, "ok[r] = TRUE" iff
    the root mean of the squared distances "d2[r+k]" is less than
    "cutDist".  The average is taken for "k" ranging in [0..minSteps]".
    
    We then exclude from the "ok" vector any pairs "(ia,ib)" where
    either "ia" or "ib" lies within a segment listed in "exSeg".
    
    The candidates are then found by identifying the maximal substrings
    of consecutive "TRUE"s in the "ok" vector.