example for walk_expression, close #501

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.17 :warning:

Comparison is base (150a154) 56.22% compared to head (d1267cd) 56.06%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #502 +/- ## ========================================== - Coverage 56.22% 56.06% -0.17% ========================================== Files 19 19 Lines 1204 1204 ========================================== - Hits 677 675 -2 - Misses 527 529 +2 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaPy/SymPy.jl/pull/502?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/lambdify.jl](https://app.codecov.io/gh/JuliaPy/SymPy.jl/pull/502?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2xhbWJkaWZ5Lmps) | `96.92% <ø> (ø)` | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/JuliaPy/SymPy.jl/pull/502/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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