Tosefta do Psalmów 136:2

Pregnant and nursing women fast on the Ninth of Av, and Yom Kippur, and on the three public fast days, but they would not fast on the remainder of the fast days. Not that they were allowed to indulge in delicacies^*ולא שיענגו את עצמן בתפנוקין (Jastrow); rather, they eat and drink as much as is required to sustain the newborn. One who was fasting for a sick person, and he recovered, [or] due to a matter of distress, and it passed -- behold, he should [nonetheless] complete his fast. Behold, one who was fasting and forgot, and ate and drank, [nonetheless] completes his fast. One who goes from a place where they are fasting to a place where they are not fasting -- behold, this one completes his fast. One who entered a city and found that they are fasting -- behold, this one fasts along with them.
What is the "Great Hallel" (see Taan. 3:9)? (Ps. 136:2-3), "Give thanks to the God of gods, [for His mercy endures forever]; Give thanks to the Lord of lords, [for His mercy endures forever]."