The affix ṭhak comes in the sense of 'who transacts business therein', after compounds ending in kaṭhina and after prastāra, being in the Locative -7th case in construction.,
Thus वंशकठिने व्यवहरति = वांशकठिनिकः 'whose occupation is in a bamboo thicket' or 'who does what is proper to be done in such a place' i.e. तस्मिन्देशे या क्रिया यथानुष्ठेया तां तथैवानुतिष्ठेति ॥ So also वार्ध्रकठिनिकः ॥ So also प्रास्तारिकः, सांस्थानिकः ॥,