According to the above-mentioned framework, the sentence "I am going to the store" is an expression, not a mathematical object. To transform it into a mathematical object, it is necessary to make some abstraction. For example, to say "I am going to x", where x is an arbitrary place. Now, the mathematical object "I am going to x" can be represented by the following set of expressions:
"I am going to the store", "I am going to the zoo", "I am going to the mountain," etc.
Any transformation of the mathematical object "I am going to x" should transform all its instantiation. For example, the transformation from "I am going to x" to "x is my destiny" is defined by the family of transformations of expressions (computations)
from "I am going to the store" to "the store is my destiny."
from "I am going to the zoo" to "the zoo is my destiny."
from "I am going to the mountain" to "the mountain is my destiny."
etc.