After a neutral aṅga (stem) a , am is substituted for su and am , the Nominative 1st-Case and Accusative -2nd case endings.,
As कुण्डं तिष्ठति, कुण्डं पश्य ॥ So also वनम्, पीठम् ॥ Why do we not say \म् is substituted\? कुण्ड + म् = कुण्डाम्, thus there would be lengthening if only म् was taught 7.3.101, while कुण्ड + अम् = कुण्डम् (the single substitution of the form of the antecedent by 6.1.107.,