The context and chaining context lookups are meta-lookups: they specify a certain context, in which other lookups may operate at different positions. The sub-lookups may be either substitution or positioning lookups.

lookup abcToxx {
    context (q) a b c (r);
    sub 0 ab_to_x;
    sub 1 c_to_x;

lookup ab_to_x {
    sub a b -> x;

lookup c_to_x {
    sub c -> x;

This example shows a (nonsense) lookup that changes the sequence "qabcr" into "qxxr". It works as follows:

Replacing i by dotlessi before marks

That was a nice example showing how things work exactly, but here is a more useful example. When the glyph "i" is followed by a mark above, the dot above the i is removed and the mark is placed over the "dotlessi" glyph. This may be done be replacing "i" by "dotlessi" whenever it is followed by a mark above.

lookup dotlessI {
    ignore mark except @marksAbove;
    context [i j] (@marksAbove);
    sub 0 iToDotlessI;

lookup iToDotlessI {
    sub i -> dotlessi;
    sub j -> dotlessj;

The context that is matched here is either i or j, followed by a combining mark above. If such a sequence is found, at position 0 (the first position of the input sequence) a lookup is applied that changes "i" into "dotlessi" and "j" into "dotlessj".

Note: OTComp will generate a contextual lookup when you specify no glyphs for the backtrack and lookahead sequences. If you do, a chaining contextual lookup will be generated.