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The order is not to sing psalms continuously in church, but to alternate them with readings.
About the fact that psalms should not be sung in a row in assemblies, but in the interval after each psalm there should be reading.
Greek original
Περὶ τοῦ μὴ δεῖν ἐπισυνάπτειν ἐν ταῖς συνάξεσι τοὺς ψαλμούς, ἀλλὰ διὰ μέσου καθ᾽ ἕκαοτον ψαλμὸν γίνεσθαι ἀνάγνωσιν.
Parallel canons
Трул.75Лаод.15