The agile approach involves iterative cycles of planning, executing, and evaluating. The act of "writing new procedural code and eliminating certain function calls" is the direct process of creating and modifying the software's source code. This activity falls squarely within the Implementation (or Development) phase, where the designs and requirements from earlier in the iteration are translated into a functional program.

A. Testing is the phase where the written code is verified to ensure it works correctly, not the writing itself.

B. Design is the phase where the program's architecture, modules, and functions are planned before coding begins.

D. Analysis is the phase focused on gathering and understanding the user requirements and project goals.

1. Pressman, R. S., & Maxim, B. R. (2020). Software Engineering: A Practitioner's Approach (9th ed.). McGraw-Hill. Chapter 4, "An Agile View of Process," describes the implementation activities within an agile iteration.

2. Fowler, M. (2005). The New Methodology. MartinFowler.com. Retrieved from https://martinfowler.com/articles/newMethodology.html (This article describes agile development, where the construction/implementation activity is a core part of each iteration.)

3. University of California, Berkeley. (n.d.). CS 169 Software Engineering Course. The course materials frequently distinguish between agile phases, placing coding activities within the implementation/development stage of a sprint.