The triangular Indexing Function
The triangular indexing function can be used to construct rtable objects of type Array or Matrix.
In the construction of a Matrix, if triangular or triangular[upper] is included in the calling sequence as an indexing function (shape), an upper triangular Matrix is returned.
This indexing function may also be qualified as either triangular[unit] or triangular[lower] (hence, also as triangular[upper, unit] and triangular[lower, unit]).
Note: The Matrix need not be square.
The specification is similar in the construction of an Array.
If an object is defined by using the triangular or triangular[upper] indexing function, the elements located in the lower triangle cannot be reassigned.
The situation is similar in a construction that uses triangular[lower] or triangular[lower, unit] as an indexing function.
M ≔ Matrix⁡3,x,y,y,z,z,z,shape=triangularlower
M1,3 ≔ 5
Error, attempt to assign to upper triangle of lower triangular rtable
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